Iconic Firefox — userChrome CSS snippet which adds back icons for most context and popup menu entries in Firefox. It uses mostly built-in icons.

iconic_firefox.css

/* Iconic Firefox (https://gist.github.com/emvaized/a379d61ed8970e5711ca7278a39c1895/) */
/* based on https://gist.github.com/qaz69wsx/83a90423163b65a8344b2e60fc356a58 (now removed) */
/* for better compatibility with dark mode, enable `svg.context-properties.content.enabled` in `about:config` */

:root{
    --uc-popup-menu-icon-color: color-mix(in srgb, currentColor 75%, transparent);
}
.subviewbutton:not(.subviewbutton-iconic, [checked="true"], [targetURI])>.toolbarbutton-icon,
.subviewbutton:not(.subviewbutton-iconic, [checked="true"], [targetURI])>.protections-popup-footer-icon{
	width:16px;
	height:16px;
	margin-inline-end:8px !important;
	-moz-context-properties:fill, fill-opacity;
	fill:var(--uc-popup-menu-icon-color);
}
menupopup:not(.in-menulist)>menu:not(.menu-iconic),
menupopup:not(.in-menulist, [aria-label])>menuitem:not(.menuitem-iconic, [checked="true"]),
menuitem[type="checkbox"],
menuitem[type="radio"],
toolbarbutton[data-l10n-id*="-manage-"],
#downloadsHistory,
#appMenu-zoom-controls{
	padding-inline-start:calc(1em + 24px) !important;
	background-position:left 1em center;
	background-repeat:no-repeat;
	background-size:16px;
	-moz-context-properties:fill, fill-opacity;
	fill:var(--uc-popup-menu-icon-color);
}
#fxa-manage-account-button>vbox>label{
	margin-inline-start:24px !important;
}
.PanelUI-remotetabs-notabsforclient-label{
	margin-inline-start:40px !important;
}
.widget-overflow-list .toolbarbutton-1:not(.toolbarbutton-combined)>.toolbarbutton-text,
toolbarbutton[data-l10n-id*="-manage-"]{
	padding-inline-start:8px !important;
}
#appMenu-fxa-label2::before,
#fxa-manage-account-button::after{
	content:"";
	display:-moz-box;
	border-radius:50%;
	background:var(--avatar-image-url) no-repeat center/contain;
	-moz-context-properties:fill, fill-opacity;
	fill:var(--uc-popup-menu-icon-color);
}
#appMenu-fxa-label2::before{
	width:16px;
	height:16px;
	margin-inline-end:8px;
}
#fxa-manage-account-button::after{
	width:32px;
	height:32px;
}
.syncNowBtn{
	visibility:visible !important;
	-moz-box-ordinal-group:0 !important;
	margin-inline-end:8px;
    order:0 !important;
}
#PanelUI-fxa-menu-setup-sync-button{
	list-style-image:url("chrome://browser/skin/sync.svg");
}
#PanelUI-fxa-menu-sendtab-button{
	list-style-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="context-fill" fill-opacity="context-fill-opacity" d="M12 0H4a2 2 0 00-2 2v2a1 1 0 001 1 1 1 0 001-1V2.5a.5.5 0 01.5-.5h7a.5.5 0 01.5.5v10a.5.5 0 01-.5.5h-7a.5.5 0 01-.5-.5V11a1 1 0 00-1-1 1 1 0 00-1 1v3a2 2 0 002 2h8a2 2 0 002-2V2a2 2 0 00-2-2zM9 15H7v-1h2z"/><path fill="context-fill" fill-opacity="context-fill-opacity" d="M5.146 10.146a.5.5 0 10.707.707l3-3a.5.5 0 000-.708l-3-3a.5.5 0 00-.707.707L7.293 7H.5a.5.5 0 000 1h6.793z"/></svg>');
}
.subviewbutton[data-l10n-id$="-settings"]:not([data-l10n-id*="-manage-"]){
	list-style-image:url("chrome://global/skin/icons/settings.svg");
}
.subviewbutton[data-l10n-id="fxa-menu-connect-another-device"]{
	list-style-image:url("chrome://global/skin/icons/plus.svg");
}
#PanelUI-fxa-menu-account-signout-button{
	list-style-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="13" height="14"><path fill="context-fill" fill-opacity="context-fill-opacity" d="M6 11.375a.875.875 0 110 1.75H3.375A2.625 2.625 0 01.75 10.5v-7A2.625 2.625 0 013.375.875H6a.875.875 0 110 1.75H3.375A.875.875 0 002.5 3.5v7c0 .483.392.875.875.875zm5.871-4.996a.875.875 0 010 1.242l-2.625 2.625a.875.875 0 01-1.242 0 .875.875 0 010-1.242L9.14 7.875H5.125a.875.875 0 110-1.75h4.016L8.004 4.996a.879.879 0 011.242-1.242z"/></svg>');
}
#appMenu-new-tab-button2{
	list-style-image:url("chrome://browser/skin/new-tab.svg");
}
#appMenu-new-window-button2,
#appMenuRecentlyClosedWindows{
	list-style-image:url("chrome://browser/skin/window.svg");
}
#appMenu-new-private-window-button2{
	list-style-image:url("chrome://browser/skin/privateBrowsing.svg");
}
.subviewbutton[data-l10n-id="library-bookmarks-menu"],
#panelMenuBookmarkThisPage[starred],
#sidebar-switcher-bookmarks{
	list-style-image:url("chrome://browser/skin/bookmark-hollow.svg");
}
#panelMenuBookmarkThisPage{
	list-style-image:url("chrome://browser/skin/bookmark-hollow.svg");
}
#panelMenu_searchBookmarks,
#appMenu-find-button2,
#appMenuSearchHistory,
#allTabsMenu-searchTabs{
	list-style-image:url("chrome://global/skin/icons/search-glass.svg");
}
#panelMenu_viewBookmarksToolbar{
	list-style-image:url("chrome://browser/skin/bookmark-star-on-tray.svg");
}
.subviewbutton[id$="-history-button"],
#sidebar-switcher-history{
	list-style-image:url("chrome://browser/skin/history.svg");
}
#appMenuRecentlyClosedTabs{
	list-style-image:url("chrome://browser/skin/tabs.svg");
}
#appMenuRestoreSession{
	list-style-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="context-fill" fill-opacity="context-fill-opacity"><path d="M13 0H3a3 3 0 00-3 3v8a3 3 0 003 3h4l-.3.4a1 1 0 101.6 1.2l1.5-2a1 1 0 000-1.2l-1.5-2a1 1 0 00-1.6 1.2l.3.4H3a1 1 0 01-1-1V5h12v6a1 1 0 01-1 1 1 1 0 000 2 3 3 0 003-3V3a3 3 0 00-3-3zM2 4V3a1 1 0 011-1h10a1 1 0 011 1v1z"/></svg>');
}
#appMenuClearRecentHistory{
	list-style-image:url("chrome://browser/skin/forget.svg");
}
.subviewbutton[id$="-downloads-button"]{
	list-style-image:url("chrome://browser/skin/downloads/downloads.svg");
}
#appMenu-passwords-button{
	list-style-image:url("chrome://browser/skin/login.svg");
}
#appMenu-extensions-themes-button{
	list-style-image:url("chrome://mozapps/skin/extensions/extension.svg");
}
#appMenu-print-button2{
	list-style-image:url("chrome://global/skin/icons/print.svg");
}
#appMenu-save-file-button2{
	list-style-image:url("chrome://browser/skin/downloads/downloads.svg");
}
.subviewbutton[command="cmd_CustomizeToolbars"]{
	list-style-image:url("chrome://browser/skin/customize.svg");
}
.subviewbutton[oncommand="switchToTabHavingURI('about:performance', true)"]{
	list-style-image:url("chrome://global/skin/icons/performance.svg");
}
.subviewbutton[key="key_browserToolbox"]{
	list-style-image:url("chrome://devtools/skin/images/tool-inspector.svg");
}
.subviewbutton[key="key_aboutProcesses"]{
	list-style-image:url("chrome://devtools/skin/images/tool-profiler.svg");
}
.subviewbutton[key="key_aboutProcesses"] + .subviewbutton{
	list-style-image:url("chrome://global/skin/icons/developer.svg");
}
.subviewbutton[key="key_browserConsole"]{
	list-style-image:url("chrome://devtools/skin/images/tool-webconsole.svg");
}
.subviewbutton[key="key_toggleToolbox"]{
	list-style-image:url("resource://devtools-shared-images/command-pick.svg");
}
.subviewbutton[key="key_viewSource"]{
	list-style-image:url("chrome://devtools/skin/images/tool-styleeditor.svg");
}
.subviewbutton[key="key_responsiveDesignMode"]:not([checked="true"]){
	list-style-image:url("chrome://devtools/skin/images/command-responsivemode.svg");
	fill-opacity:0;
}
.subviewbutton[key="key_responsiveDesignMode"]+.subviewbutton{
	list-style-image:url("chrome://devtools/skin/images/command-eyedropper.svg");
}
.subviewbutton[data-l10n-id="appmenu-developer-tools-extensions"]{
	list-style-image:url("chrome://activity-stream/content/data/content/assets/glyph-webextension-16.svg");
}
#appMenu-help-button2,
#appMenu_menu_openHelp
{
	list-style-image:url("chrome://global/skin/icons/help.svg");
}
#appMenu_menu_HelpPopup_reportPhishingtoolmenu{
	list-style-image:url("chrome://global/skin/icons/warning.svg");
}
#appMenu_feedbackPage{
	list-style-image:url("chrome://global/skin/icons/lightbulb.svg");
}
#appMenu_troubleShooting{
	list-style-image:url("chrome://devtools/skin/images/browsers/firefox.svg");
}
#appMenu_helpSafeMode{
	list-style-image:url("chrome://global/skin/icons/security.svg");
}
#appMenu_helpSwitchDevice {
	list-style-image:url("chrome://browser/skin/import.svg");
}
#appMenu_aboutName{
	list-style-image:url("chrome://global/skin/icons/info.svg");
}
#appMenu-quit-button2{
	list-style-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="context-fill" fill-opacity="context-fill-opacity" d="M8 6a1 1 0 001-1V1a1 1 0 00-2 0v4a1 1 0 001 1zm3.5-4.032a1 1 0 00-1 1.732A4.946 4.946 0 0113 8 5 5 0 013 8a4.946 4.946 0 012.5-4.3 1 1 0 00-1-1.732 7 7 0 107.006 0z"/></svg>');
}
#allTabsMenu-containerTabsButton{
	list-style-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="context-fill"><rect width="6" height="6" x="1" y="1" rx="1"/><path d="M14.75 3H13V1.25a.25.25 0 00-.25-.25h-1.5a.25.25 0 00-.25.25V3H9.25a.25.25 0 00-.25.25v1.5a.25.25 0 00.25.25H11v1.75a.25.25 0 00.25.25h1.5a.25.25 0 00.25-.25V5h1.75a.25.25 0 00.25-.25v-1.5a.25.25 0 00-.25-.25z"/><rect width="6" height="6" x="1" y="9" rx="1"/><rect width="6" height="6" x="9" y="9" rx="1"/></svg>');
}
#sidebar-switcher-tabs{
	list-style-image:url("chrome://browser/skin/tab.svg");
}
#sidebar-reverse-position[label="Move Sidebar to Right"]{
	list-style-image:url("chrome://browser/skin/sidebars-right.svg");
}
#sidebar-reverse-position[label="Move Sidebar to Left"]{
	list-style-image:url("chrome://browser/skin/sidebars.svg");
}
.subviewbutton[data-l10n-id="sidebar-menu-close"]{
	list-style-image:url("chrome://global/skin/icons/close.svg");
}
#BMB_bookmarksPopup menuitem:not(.menuitem-iconic),
#PlacesToolbar .openintabs-menuitem{
	padding-inline-start:32px !important;
	background-position-x:8px;
}
#menu_newNavigatorTab,
menuitem[id$="openANewTab"]{
	background-image:url("chrome://browser/skin/new-tab.svg");
}
#menu_newUserContext{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="context-fill"><rect width="6" height="6" x="1" y="1" rx="1"/><path d="M14.75 3H13V1.25a.25.25 0 00-.25-.25h-1.5a.25.25 0 00-.25.25V3H9.25a.25.25 0 00-.25.25v1.5a.25.25 0 00.25.25H11v1.75a.25.25 0 00.25.25h1.5a.25.25 0 00.25-.25V5h1.75a.25.25 0 00.25-.25v-1.5a.25.25 0 00-.25-.25z"/><rect width="6" height="6" x="1" y="9" rx="1"/><rect width="6" height="6" x="9" y="9" rx="1"/></svg>');
}
#menu_newNavigator,
#historyUndoWindowMenu,
#context_openTabInWindow,
menuitem[data-l10n-id*="-open-"][data-l10n-id$="-window"]{
	background-image:url("chrome://browser/skin/window.svg");
}
menuitem[data-l10n-id$="-private-window"]{
	background-image:url("chrome://browser/skin/privateBrowsing.svg") !important;
}
#menu_openFile{
	background-image:url("chrome://browser/skin/open.svg");
}
#menu_savePage,
#context-saveimage,
#context-savelink,
#context-saveframe,
#context-savevideo,
#context-savepage{
	background-image:url("chrome://browser/skin/downloads/downloads.svg");
}
menuitem[data-l10n-id*="-email"]{
	background-image:url("chrome://browser/skin/mail.svg");
}
menuitem[data-l10n-id*="-print"]{
	background-image:url("chrome://global/skin/icons/print.svg");
}
#menu_importFromAnotherBrowser,
#browserImport{
	background-image:url("chrome://browser/skin/import.svg");
}
#menu_FileQuitItem{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="context-fill" fill-opacity="context-fill-opacity" d="M8 6a1 1 0 001-1V1a1 1 0 00-2 0v4a1 1 0 001 1zm3.5-4.032a1 1 0 00-1 1.732A4.946 4.946 0 0113 8 5 5 0 013 8a4.946 4.946 0 012.5-4.3 1 1 0 00-1-1.732 7 7 0 107.006 0z"/></svg>');
}
menuitem[data-l10n-id="text-action-undo"],
menuitem[data-l10n-id$="-reopen-closed-tabs"]{
	background-image:url("chrome://global/skin/icons/undo.svg");
}
menuitem[data-l10n-id="text-action-redo"]{
	background-image:url('data:image/svg+xml,<svg version="1.1" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg"><path d="m1 11c0 0.55228 0.44772 1 1 1s1-0.44772 1-1c0-2.7614 2.2386-5 5-5 2.0503 0 3.8124 1.2341 4.584 3h-2.5883c-0.55228 0-1 0.44772-1 1 0 0.55228 0.44772 1 1 1h5.0043c0.55228 0 1-0.44772 1-1v-5c0-0.55228-0.44772-1-1-1s-1 0.44772-1 1v2.3924c-1.2249-2.0328-3.4537-3.3924-6-3.3924-3.866 0-7 3.134-7 7z" fill="context-fill"/></svg>');
}
menuitem[data-l10n-id="text-action-cut"]{
	background-image:url("chrome://browser/skin/edit-cut.svg");
}
menuitem[data-l10n-id="text-action-copy"],
#context-copyimage-contents,
#syncedTabsCopySelected{
	background-image:url("chrome://global/skin/icons/edit-copy.svg");
}
#context-stripOnShareLink,
menuitem[data-l10n-id="text-action-strip-on-share"]{
	background-image:url("chrome://browser/skin/fingerprint.svg");
}
menuitem[data-l10n-id="text-action-paste"]{
	/* background-image:url("chrome://browser/skin/edit-paste.svg"); */
	background-image:url('data:image/svg+xml,<svg width="16" height="16" version="1.1" xmlns="http://www.w3.org/2000/svg"><path d="m5.5583 7.0446h4.9833v1.3338h-4.9833zm0 3.0486h3.0437v1.3338h-3.0437z" fill="context-fill" fill-rule="evenodd" stroke-width=".76441"/><path d="m11.883 1.5189h-1.1807a3.0667 3.0486 0 0 0-5.3053 0h-1.1807a1.9167 1.9054 0 0 0-1.9167 1.9054v10.67a1.9167 1.9054 0 0 0 1.9167 1.9054h7.6667a1.9167 1.9054 0 0 0 1.9167-1.9054v-10.67a1.9167 1.9054 0 0 0-1.9167-1.9054zm-5.75 3.8108a1.15 1.1432 0 0 1-1.15-1.1432v-1.3338h-0.76667a0.575 0.57162 0 0 0-0.575 0.57162v10.67c0 0.31249 0.26067 0.57162 0.575 0.57162h7.6667c0.31433 0 0.575-0.25914 0.575-0.57162v-10.67a0.575 0.57162 0 0 0-0.575-0.57162h-0.76667v1.3338c0 0.63259-0.51367 1.1432-1.15 1.1432zm1.15-1.9054v-1.5243h1.5333v1.5243z" fill="context-fill" fill-rule="evenodd" stroke-width=".76441"/></svg>');
}
menuitem[data-l10n-id="text-action-paste-no-formatting"]{
	background-image:url("chrome://browser/skin/edit-paste.svg");
}
menuitem[data-l10n-id="text-action-delete"],
.customize-context-removeExtension,
menuitem[data-l10n-id="toolbar-context-menu-remove-from-toolbar"],
.downloadDeleteFileMenuItem,
menuitem[id^="placesContext_delete"]{
	background-image:url("chrome://global/skin/icons/delete.svg");
}
#menu_find,
#BMB_searchBookmarks,
#context-searchselect-private,
#context-searchselect,
#context-keywordfield{
	background-image:url("chrome://global/skin/icons/search-glass.svg");
}
#toggle_PersonalToolbar,
#BMB_viewBookmarksToolbar{
	background-image:url("chrome://browser/skin/bookmarks-toolbar.svg");
}
menuitem[data-l10n-id*="-customize-toolbar"]{
	background-image:url("chrome://browser/skin/customize.svg");
}
#viewSidebarMenuMenu,
#BMB_viewBookmarksSidebar{
	background-image:url("chrome://browser/skin/sidebars.svg");
}
#menu_bookmarksSidebar:not([checked="true"]),
menuitem[data-l10n-id="menu-bookmark-edit"]{
	background-image:url("chrome://browser/skin/bookmark.svg") !important;
}
#menu_historySidebar:not([checked="true"]){
	background-image:url("chrome://browser/skin/history.svg");
}
#menu_tabsSidebar:not([checked="true"]),
#sync-tabs-menuitem{
	background-image:url("chrome://browser/skin/tab.svg");
}
#menu_zoomEnlarge{
	background-image:url("chrome://browser/skin/add-circle-fill.svg");
}
#menu_zoomReduce{
	background-image:url("chrome://browser/skin/subtract-circle-fill.svg");
}
#repair-text-encoding{
	background-image:url("chrome://browser/skin/characterEncoding.svg");
}
#fullScreenItem:not([checked="true"]){
	background-image:url("chrome://browser/skin/fullscreen.svg");
}
#menu_readerModeItem{
	background-image:url("chrome://browser/skin/reader-mode.svg");
}
#sanitizeItem,
#placesContext_deleteHost{
	background-image:url("chrome://browser/skin/forget.svg");
}
#historyRestoreLastSession{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="context-fill" fill-opacity="context-fill-opacity"><path d="M13 0H3a3 3 0 00-3 3v8a3 3 0 003 3h4l-.3.4a1 1 0 101.6 1.2l1.5-2a1 1 0 000-1.2l-1.5-2a1 1 0 00-1.6 1.2l.3.4H3a1 1 0 01-1-1V5h12v6a1 1 0 01-1 1 1 1 0 000 2 3 3 0 003-3V3a3 3 0 00-3-3zM2 4V3a1 1 0 011-1h10a1 1 0 011 1v1z"/></svg>');
}
#historyUndoMenu{
	background-image:url("chrome://devtools/skin/images/debugging-tabs.svg");
}
menuitem[data-l10n-id*="bookmark-"]:not(.menuitem-iconic),
#context-bookmarkframe,
#placesContext_createBookmark{
	background-image:url("chrome://browser/skin/bookmark-hollow.svg");
}
#menu_openDownloads{
	background-image:url("chrome://browser/skin/downloads/downloads.svg");
}
#menu_openAddons{
	background-image:url("chrome://mozapps/skin/extensions/extension.svg");
}
#sync-setup{
	background-image:var(--avatar-image-url);
}
menuitem[data-l10n-id$="-sync-now"]{
	background-image:url("chrome://browser/skin/sync.svg");
}
#webDeveloperMenu{
	background-image:url("chrome://browser/skin/developer.svg");
}
#menu_taskManager{
	background-image:url("chrome://global/skin/icons/performance.svg");
}
#menu_browserToolbox{
	background-image:url("chrome://devtools/skin/images/fox-smiling.svg");
}
#menu_responsiveUI:not([checked="true"]){
	background-image:url("chrome://devtools/skin/images/command-responsivemode.svg");
	fill-opacity:0;
}
#menu_eyedropper{
	background-image:url("chrome://devtools/skin/images/command-eyedropper.svg");
}
#extensionsForDevelopers{
	background-image:url("chrome://activity-stream/content/data/content/assets/glyph-webextension-16.svg");
}
#menu_pageInfo,
menuitem[data-l10n-id*="about-"],
#context-viewimageinfo,
#context-viewframeinfo{
	background-image:url("chrome://global/skin/icons/info.svg");
}
#menu_preferences,
#openSettingsMenuItem{
	background-image:url("chrome://global/skin/icons/settings.svg");
}
#menu_HelpPopup_reportPhishingtoolmenu{
	background-image:url("chrome://global/skin/icons/lightbulb.svg");
}
#aboutName{
	background-image:url("chrome://devtools/skin/images/browsers/firefox.svg");
}
menuitem[data-l10n-id*="reload-"],
#context-reloadframe{
	background-image:url("chrome://global/skin/icons/reload.svg");
}
menuitem[id^="context_toggleMute"]:not([soundplaying], [muted]),
menuitem[id^="context_toggleMute"][soundplaying]:not([muted]),
#context-media-mute{
	background-image:url("chrome://global/skin/media/audio-muted.svg");
}
menuitem[id^="context_toggleMute"][muted],
#context-media-unmute{
	background-image:url("chrome://global/skin/media/audio.svg");
}
menuitem[data-l10n-id^="pin-"],
.customize-context-moveToPanel{
	background-image:url("chrome://activity-stream/content/data/content/assets/glyph-pin-16.svg");
}
menuitem[data-l10n-id^="unpin-"],
.customize-context-moveToToolbar{
	background-image:url("chrome://activity-stream/content/data/content/assets/glyph-unpin-16.svg");
}
menuitem[id^="context_duplicateTab"]{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="context-fill" fill-opacity="context-fill-opacity"><path d="M15 13a1 1 0 000-2h-1V5a2 2 0 00-2-2H8.402a2 2 0 00-2 2v6h-1a1 1 0 000 2"/><path d="M5.281 10V5c0-.771.301-1.468.78-2H4a2 2 0 00-2 2v6H1a1 1 0 000 2h2.559a1.978 1.978 0 01-.278-1c0-1.103.897-2 2-2z"/></svg>');
}
#context_moveTabOptions{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="context-fill" fill-opacity="context-fill-opacity"><path d="M15.854 12.14s-.001 0 0 0l-3.001-3.001a.496.496 0 00-.707.013.5.5 0 000 .695l2.147 2.148H7.5a.5.5 0 000 1h6.793l-2.147 2.146a.5.5 0 10.695.719l.012-.012 3-3a.5.5 0 00.001-.708z"/><path d="M7.5 10.994h4.38l-.453-.453a1.495 1.495 0 010-2.084 1.503 1.503 0 012.133-.025l.44.44V5a2 2 0 00-2-2H4a2 2 0 00-2 2v6H1a1 1 0 000 2h5.094A1.49 1.49 0 016 12.494c0-.827.673-1.5 1.5-1.5z"/></svg>');
}
menu.sync-ui-item{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="context-fill" fill-opacity="context-fill-opacity" d="M12 0H4a2 2 0 00-2 2v2a1 1 0 001 1 1 1 0 001-1V2.5a.5.5 0 01.5-.5h7a.5.5 0 01.5.5v10a.5.5 0 01-.5.5h-7a.5.5 0 01-.5-.5V11a1 1 0 00-1-1 1 1 0 00-1 1v3a2 2 0 002 2h8a2 2 0 002-2V2a2 2 0 00-2-2zM9 15H7v-1h2z"/><path fill="context-fill" fill-opacity="context-fill-opacity" d="M5.146 10.146a.5.5 0 10.707.707l3-3a.5.5 0 000-.708l-3-3a.5.5 0 00-.707.707L7.293 7H.5a.5.5 0 000 1h6.793z"/></svg>');
}
menuitem.sendtab-target[clientType="desktop"]{
	background-image:url("chrome://browser/skin/device-desktop.svg");
}
menuitem.sendtab-target[clientType="phone"]{
	background-image:url("chrome://browser/skin/device-phone.svg");
}
menuitem.sendtab-target[clientType="tablet"]{
	background-image:url("chrome://browser/skin/device-tablet.svg");
}
menuitem.sendtab-target[clientType="tv"]{
	background-image:url("chrome://browser/skin/device-tv.svg");
}
menuitem.sendtab-target[clientType="vr"]{
	background-image:url("chrome://browser/skin/device-vr.svg");
}
.share-tab-url-item{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="context-fill" d="M12.707 4.294l-4-4A1 1 0 008.38.077a.984.984 0 00-.246-.05A.938.938 0 008 0a.938.938 0 00-.134.027.984.984 0 00-.246.05A1 1 0 007.291.3l-4 4a1 1 0 001.416 1.408L7 3.415V11a1 1 0 002 0V3.415l2.293 2.293a1 1 0 001.414-1.414z"/><path fill="context-fill" d="M14 9a1 1 0 00-1 1v3a1 1 0 01-1 1H4a1 1 0 01-1-1v-3a1 1 0 00-2 0v3a3 3 0 003 3h8a3 3 0 003-3v-3a1 1 0 00-1-1z"/></svg>');
}
#context_reopenInContainer,
#context-openlinkinusercontext-menu{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="context-fill"><rect width="6" height="6" x="1" y="1" rx="1"/><rect width="6" height="6" x="1" y="9" rx="1"/><rect width="6" height="6" x="9" y="9" rx="1"/><path d="M14.92 1.62a1 1 0 00-.54-.54A1 1 0 0014 1h-4a1 1 0 000 2h1.59l-2.3 2.29a1 1 0 000 1.42 1 1 0 001.42 0L13 4.41V6a1 1 0 002 0V2a1 1 0 00-.08-.38z"/></svg>');
}
#context_closeTab,
#orgClose{
	background-image:url("chrome://global/skin/icons/close.svg");
}
#context_closeTabOptions{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="context-fill" fill-opacity="context-fill-opacity" d="M14.3 10l-1.8 1.8-1.8-1.8c-.2-.2-.5-.2-.7 0s-.2.5 0 .7l1.8 1.8-1.8 1.8c-.2.2-.2.5 0 .7s.5.2.7 0l1.8-1.8 1.8 1.8c.2.2.5.2.7 0s.2-.5 0-.7l-1.8-1.8 1.8-1.8c.2-.2.2-.5 0-.7s-.5-.2-.7 0z"/><path fill="context-fill" fill-opacity="context-fill-opacity" d="M16 12c0-.2-.1-.5-.2-.6l-1.1 1.1.5.5c.4-.1.8-.5.8-1zm-5.6.5L9 11.1c-.4-.4-.4-1 0-1.4l.7-.7c.4-.4 1-.4 1.4 0l1.4 1.4L13.9 9s.1 0 .1-.1V5c0-1.1-.9-2-2-2H4c-1.1 0-2 .9-2 2v6H1c-.6 0-1 .4-1 1s.4 1 1 1h8.9l.5-.5z"/></svg>');
}
menuitem[data-l10n-id="full-screen-exit"]{
	background-image:url("chrome://browser/skin/fullscreen-exit.svg");
}
#paste-and-go{
	background-image:url("chrome://browser/skin/forward.svg");
}
.customize-context-reportExtension{
	background-image:url("chrome://global/skin/icons/warning.svg");
}
.downloadShowMenuItem,
#placesContext_showInFolder{
	background-image:url("chrome://global/skin/icons/folder.svg");
}
.openintabs-menuitem,
menuitem[id^="context-open"]:is([id$="intab"], [id$="incontainertab"]),
menuitem[data-l10n-id$="-view-new-tab"],
menuitem[data-l10n-id*="-open-in-"][data-l10n-id$="-tab"]{
	background-image:url("chrome://global/skin/icons/open-in-new.svg");
}
#context-pocket,
#context-savelinktopocket{
	background-image:url("chrome://global/skin/icons/pocket-outline.svg");
}
menuitem[data-l10n-id$="-screenshot"]{
	background-image:url("chrome://browser/skin/screenshot.svg");
}
menuitem[data-l10n-id*="-copy-"][data-l10n-id*="-link"]{
	/* background-image:url("chrome://global/skin/icons/link.svg"); */
	background-image:url('data:image/svg+xml,<svg width="16" height="16" fill="none" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg"><path d="m2.6947 9.0231c0.3841 0.38405 0.88022 0.57615 1.3844 0.57615l0.00801 0.0079c0.49611 0 1.0002-0.19202 1.3843-0.57607l0.30407-0.30402-0.99224-0.99204-0.30408 0.30404c-0.20806 0.21603-0.57614 0.21603-0.79219 0l-2.1125-2.1123c-0.23207-0.23196-0.23207-0.61609 0-0.84805l3.5049-3.5044c0.23207-0.23206 0.61614-0.23206 0.8482 0l2.1125 2.1123c0.11203 0.10401 0.16804 0.23997 0.16804 0.39204 0 0.15197-0.064016 0.29603-0.16804 0.40005l-0.30408 0.30404 0.99224 0.99211 0.30408-0.30404c0.76819-0.76817 0.76819-2.0083 0-2.7763l-2.1125-2.1123c-0.78416-0.77617-2.0485-0.77617-2.8327 0l-3.5049 3.4964c-0.77619 0.78408-0.77619 2.0482 0 2.8323z" fill="context-fill"/> <path d="m5.6523 4.6555-0.99018 0.99006 5.6865 5.6858 0.99024-0.99004z" fill="context-fill"/><path d="m10.497 16c-0.51213 0-1.0243-0.19202-1.4163-0.58399l-2.1125-2.1123c-0.7682-0.76801-0.7682-2.0082 0-2.7763l0.30408-0.30404 0.99224 0.99212-0.30408 0.30404c-0.10403 0.10402-0.16804 0.24795-0.16804 0.40005 0 0.15194 0.056013 0.28796 0.16804 0.39197l2.1125 2.1123c0.23206 0.23195 0.61615 0.23195 0.84821 0l3.5049-3.5044c0.23206-0.23211 0.23206-0.61616 0-0.84818l-2.1125-2.1123c-0.20805-0.21603-0.58414-0.20803-0.7922 0l-0.30408 0.30404-0.99224-0.99212 0.30408-0.30404c0.76819-0.76807 2.0085-0.76807 2.7767 0l2.1125 2.1123c0.77619 0.7841 0.77619 2.0483 0 2.8323l-3.5049 3.5045c-0.3921 0.39197-0.90422 0.58399-1.4163 0.58399z" fill="context-fill"/></svg>');
}
#context-searchselect[label*="Google"]{
	background-image:url("chrome://activity-stream/content/data/content/tippytop/favicons/google-com.ico");
}
#context-searchselect[label*="Bing"]{
	background-image:url("chrome://activity-stream/content/data/content/tippytop/favicons/bing-com.ico");
}
#context-searchselect[label*="DuckDuckGo"]{
	background-image:url("chrome://activity-stream/content/data/content/tippytop/favicons/duckduckgo-com.ico");
}
#context-inspect-a11y{
	background-image:url("chrome://devtools/skin/images/tool-accessibility.svg");
}
#context-inspect{
	background-image:url("chrome://devtools/content/shared/images/command-pick.svg");
}
#context-media-play{
	background-image:url("chrome://global/skin/media/play-fill.svg");
}
#context-media-pause,
#doNotDisturbMenuItem{
	background-image:url("chrome://global/skin/media/pause-fill.svg");
}
#context-video-fullscreen{
	background-image:url("chrome://browser/skin/fullscreen.svg");
}
#context-leave-dom-fullscreen{
	background-image:url("chrome://browser/skin/fullscreen-exit.svg");
}
#context-video-saveimage{
	background-image:url("chrome://devtools/skin/images/command-screenshot.svg");
}
#disableForOriginMenuItem{
	background-image:url("chrome://browser/skin/notification-icons/desktop-notification-blocked.svg");
}
menuitem[data-l10n-id^="places-edit-"]{
	background-image:url("chrome://global/skin/icons/edit.svg");
}
#context-translate-selection{
	background-image:url("chrome://browser/skin/translations.svg");
}
#appMenu-translate-button{
	list-style-image:url("chrome://browser/skin/translations.svg");
}
#appMenu-more-button2,
#identity-popup-more-info{
	list-style-image:url("chrome://global/skin/icons/more.svg");
}
#context-viewsource,
#context-viewframesource,
#context-viewpartialsource-selection{
	background-image:url("chrome://devtools/skin/images/tool-styleeditor.svg");
}
#context-inspect{
	background-image:url("resource://devtools-shared-images/command-pick.svg");
}
#context-selectall,
menuitem[data-l10n-id="text-action-select-all"]{
	background-image:url('data:image/svg+xml,<svg fill="context-fill" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg"><path d="m2.1667 0.5c-0.92046 0-1.6667 0.7462-1.6667 1.6667v1.6667h3.3334v-3.3334zm-1.6667 5.8333v3.3334h3.3334v-3.3334zm0 7.5v-1.6667h3.3334v3.3334h-1.6667c-0.92046 0-1.6667-0.74616-1.6667-1.6667zm9.1666 1.6667h-3.3334v-3.3334h3.3334zm2.5 0h1.6667c0.9205 0 1.6667-0.74616 1.6667-1.6667v-1.6667h-3.3334zm3.3334-9.1666v3.3334h-3.3334v-3.3334zm0-2.5v-1.6667c0-0.92046-0.74616-1.6667-1.6667-1.6667h-1.6667v3.3334zm-5.8333-3.3334h-3.3334v3.3334h3.3334z" stroke-width=".93749"/></svg>');
}
#unified-extensions-context-menu-move-widget-up{
	background-image:url("chrome://global/skin/icons/arrow-up.svg");
}
#unified-extensions-context-menu-move-widget-down{
	background-image:url("chrome://global/skin/icons/arrow-down.svg");
}
.customize-context-manageExtension,
#unified-extensions-context-menu-manage-extension,
menuitem.sendtab-target:last-child{
	background-image:url("chrome://global/skin/icons/settings.svg");
}
#unified-extensions-context-menu-remove-extension{
	background-image:url("chrome://global/skin/icons/delete.svg");
}
#unified-extensions-context-menu-report-extension{
	background-image:url("chrome://global/skin/icons/warning.svg");
}
#context-setDesktopBackground{
	background-image:url("chrome://browser/skin/canvas.svg");
}
#appMenu-zoom-controls{
	background-image:url("chrome://global/skin/media/fullscreenEnterButton.svg");
	margin-inline-start:0px !important;
}
menu#frame{
	background-image:url("chrome://browser/skin/window.svg");
}
#context_moveToStart,
#context_closeTabsToTheStart{
	background-image:url("chrome://global/skin/icons/arrow-left.svg");
}
#context_moveToEnd,
#context_closeTabsToTheEnd{
	background-image:url("chrome://global/skin/icons/arrow-right.svg");
}
#context-media-playbackrate{
	background-image:url("chrome://global/skin/icons/performance.svg");
}
#identity-popup-clear-sitedata-button{
	list-style-image:url("chrome://global/skin/icons/delete.svg");
}
#toolbar-context-remove-from-toolbar{
	background-image:url("chrome://global/skin/icons/close.svg");
}
#context-showonlythisframe,
#context-media-showcontrols{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16" fill="context-fill" fill-opacity="context-fill-opacity"><path d="M16 7.595C14.675 4.244 11.493 2 8 2S1.325 4.244 0 7.595l0 .809C1.325 11.756 4.507 14 8 14s6.675-2.244 8-5.595l0-.81zM8 12.75c-3.013 0-5.669-1.856-6.83-4.75C2.331 5.106 4.987 3.25 8 3.25S13.669 5.106 14.83 8c-1.161 2.894-3.817 4.75-6.83 4.75z"/><path d="M8 11c-1.654 0-3-1.346-3-3s1.346-3 3-3 3 1.346 3 3-1.346 3-3 3zm0-4.75c-.965 0-1.75.785-1.75 1.75S7.035 9.75 8 9.75 9.75 8.965 9.75 8 8.965 6.25 8 6.25z"/></svg>');
}
#context-media-hidecontrols{
	background-image:url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16" fill="context-fill" fill-opacity="context-fill-opacity"><path d="M3.067 1.183a.626.626 0 0 0-.885.885l1.306 1.306A8.885 8.885 0 0 0 0 7.595l0 .809C1.325 11.756 4.507 14 8 14c1.687 0 3.294-.535 4.66-1.455l2.273 2.273a.626.626 0 0 0 .884-.886L3.067 1.183zm3.759 5.528 2.463 2.463c-.32.352-.777.576-1.289.576-.965 0-1.75-.785-1.75-1.75 0-.512.225-.969.576-1.289zM8 12.75c-3.013 0-5.669-1.856-6.83-4.75a7.573 7.573 0 0 1 3.201-3.745l1.577 1.577A2.958 2.958 0 0 0 5 8c0 1.654 1.346 3 3 3 .858 0 1.624-.367 2.168-.948l1.613 1.613A7.118 7.118 0 0 1 8 12.75z"/><path d="M8 2c-.687 0-1.356.11-2.007.275l1.049 1.049A7.06 7.06 0 0 1 8 3.25c3.013 0 5.669 1.856 6.83 4.75a7.925 7.925 0 0 1-1.141 1.971l.863.863A9.017 9.017 0 0 0 16 8.404l0-.809C14.675 4.244 11.493 2 8 2z"/></svg>');
}
menuitem[type="checkbox"],
menuitem[type="radio"]{
	/* background-image:url("chrome://devtools/skin/images/checkbox.svg"); */
	background-image:url('data:image/svg+xml,<svg width="16" height="16" fill="context-fill" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg"><path d="m2.6667 0h10.667a2.6667 2.6667 0 0 1 2.6667 2.6667v10.667a2.6667 2.6667 0 0 1-2.6667 2.6667h-10.667a2.6667 2.6667 0 0 1-2.6667-2.6667v-10.667c0-1.4667 1.2-2.6667 2.6667-2.6667zm0 1.3333a1.3333 1.3333 0 0 0-1.3333 1.3333v10.667c0 0.8 0.53333 1.3333 1.3333 1.3333h10.667c0.8 0 1.3333-0.53333 1.3333-1.3333v-10.667c0-0.8-0.53333-1.3333-1.3333-1.3333z"/></svg>');
	fill: color-mix(in srgb, var(--uc-popup-menu-icon-color) 66%, transparent) !important;
	padding-inline-start:1em !important;
}
#context-video-pictureinpicture:not([checked="true"]){
	background-image:url("chrome://global/skin/media/picture-in-picture-open.svg");
	fill: var(--uc-popup-menu-icon-color) !important;
}
#spell-dictionaries{
	background-image:url("chrome://devtools/skin/images/globe.svg");
}
#spell-add-dictionaries{
	background-image:url("chrome://global/skin/icons/plus.svg");
}
toolbarbutton[data-l10n-id*="-manage-"],
#manage-saved-logins{
	background-image:url("chrome://browser/skin/menu.svg");
}
#context-sendpagetodevice-popup menuseparator + menuitem{
	background-image:url("chrome://global/skin/icons/chevron.svg");
}
#downloadsHistory{
	background-image:url("chrome://browser/skin/downloads/downloads.svg");
}
.downloadRemoveFromHistoryMenuItem{
	background-image:url("chrome://global/skin/icons/close.svg")
}
.downloadOpenReferrerMenuItem{
	background-image:url("chrome://global/skin/icons/defaultFavicon.svg")
}
menuitem[data-l10n-id="downloads-cmd-clear-list"]{
	background-image:url("chrome://browser/skin/forget.svg");
}
#protections-popup-settings-button{
	list-style-image:url("chrome://global/skin/icons/settings.svg");
}
#protections-popup-show-report-button{
	list-style-image:url("chrome://browser/skin/controlcenter/dashboard.svg");
}
toolbarbutton.restoreallitem{
	list-style-image:url("chrome://browser/skin/forget.svg");
}
toolbarbutton.restoreallitem > .toolbarbutton-icon{
	transform:scaleX(-1);
}
#appMenu-report-broken-site-button,
#protections-popup-report-broken-site-button{
	list-style-image:url("chrome://global/skin/icons/error.svg");
}
#context-ask-chat{
	background-image:url("chrome://global/skin/icons/highlights.svg");
}
#fill-login{
	background-image:url("chrome://browser/skin/login.svg");
}
menuitem[id="context-openlinkinsidebar"][label="Open Link in Second Sidebar"]{
	background-image:url("chrome://userscripts/content/second_sidebar/icons/sidebar-right.svg");
}
#allTabsMenu-closeDuplicateTabs{
	list-style-image:url('data:image/svg+xml,<svg width="16" height="16" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg"><path d="m10.322 12.666v-1.3333h-8.4443c-0.24444 0-0.44444-0.2-0.44444-0.44444v-7.111c0-0.24444 0.2-0.44444 0.44444-0.44444h3.9999v1.7777c0 0.24533 0.19911 0.44444 0.44444 0.44444h6.6666v3.1111h1.3333v-4.8888c0-0.98221-0.79554-1.7777-1.7777-1.7777h-10.666c-0.98221 0-1.7777 0.79554-1.7777 1.7777v7.111c0 0.98221 0.79554 1.7777 1.7777 1.7777z" fill="context-fill"/><path d="m16.1 10.942-1.2797 1.2797 1.2797 1.2797-0.94278 0.94278-1.2797-1.2797-1.2797 1.2797-0.94278-0.94278 1.2797-1.2797-1.2797-1.2797 0.94278-0.94278 1.2797 1.2797 1.2797-1.2797z" fill="context-fill"/></svg>');
}
[data-l10n-id="places-add-separator"] {
	background-image:url("chrome://global/skin/reader/content-width-20.svg");
}
[data-l10n-id="places-add-bookmark"] {
	background-image: url('chrome://global/skin/icons/plus.svg');
}
[data-l10n-id="places-add-folder-contextmenu"] {
	background-image:url('data:image/svg+xml,<svg width="16" height="16" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg"><path d="m1.7391 2.7174c-0.28783 0-0.52174 0.23391-0.52174 0.52174v8.3478c0 0.28787 0.23391 0.52174 0.52174 0.52174h7.6522v1.2174h-7.6522c-0.96017 0-1.7391-0.77899-1.7391-1.7391v-8.3478c0-0.96017 0.77896-1.7391 1.7391-1.7391h3.1381c0.46303 0 0.90655 0.1841 1.2332 0.51291l1.5653 1.574h4.4982c0.96014 0 1.7391 0.77896 1.7391 1.7391v3.5652h-1.2174v-3.5652c0-0.28783-0.23388-0.52174-0.52174-0.52174h-4.7513c-0.16202 0-0.31736-0.064601-0.43158-0.17949l-1.744-1.7537c-0.097683-0.098323-0.23037-0.15373-0.36978-0.15373z" fill="context-fill"/><path d="m12.609 13.326h-2.1739v-1.2174h2.1739v-2.1739h1.2174v2.1739h2.1739v1.2174h-2.1739v2.1739h-1.2174z" fill="context-fill"/></svg>');
}

menupopup[needsgutter] {
    @media (-moz-platform: linux) or (-moz-platform: windows) {
        > :is(menu, menuitem):not([image], [checked="true"]) > .menu-icon {
            display: none !important;
        }
    }
}

Just a heads-up:
The following two resources will be removed starting from Firefox version 140:

• resource://gre-resources/password.svg
• resource://gre-resources/password-hide.svg

I'm not sure if Firefox still includes these files elsewhere, but if not, it might be a good idea to extract and store the SVG content ahead of time for future use.

@yuuqilin
Thanks for the heads-up! I updated these icons to use svg icons directly.

Looks like starting from Firefox 140, there's a change in how the menu gutter works:
https://bugzilla.mozilla.org/show_bug.cgi?id=1966100

Because of that, menupopup[needsgutter] ends up with extra space where the (empty) .menu-icon is shown, which wasn't an issue before. I ran into this too, and adding the following seems to fix it:

menupopup[needsgutter] {
    @media (-moz-platform: linux) or (-moz-platform: windows) {
        > :is(menu, menuitem):not([image]) > .menu-icon {
            display: none !important;
        }
    }
}

Might be worth including in the style. Hope this helps!

@yuuqilin
Thanks for suggestion, I'll update the script!

It also seems to have changed how menuitem[type="checkbox"][checked="true] operates. I'm thinking now how to handle this problem as well.

With padding-inline-start:1em !important; rule commented out for menuitem[type="checkbox"] the padding is fixed, but the checkbox for the checked item doesn't appear:

You can try the following style to see if it fixes the issue:

menupopup[needsgutter] {
    @media (-moz-platform: linux) or (-moz-platform: windows) {
        > :is(menu, menuitem):not([image], [checked="true"]) > .menu-icon {
            display: none !important;
        }
    }
}

menupopup:not(.in-menulist, [aria-label]) > menuitem[checked="true"] {
    padding-inline-start: 1em !important;
    background-image: url('data:image/svg+xml,<svg width="16" height="16" fill="context-fill" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg"><path d="m2.6667 0h10.667a2.6667 2.6667 0 0 1 2.6667 2.6667v10.667a2.6667 2.6667 0 0 1-2.6667 2.6667h-10.667a2.6667 2.6667 0 0 1-2.6667-2.6667v-10.667c0-1.4667 1.2-2.6667 2.6667-2.6667zm0 1.3333a1.3333 1.3333 0 0 0-1.3333 1.3333v10.667c0 0.8 0.53333 1.3333 1.3333 1.3333h10.667c0.8 0 1.3333-0.53333 1.3333-1.3333v-10.667c0-0.8-0.53333-1.3333-1.3333-1.3333z"/></svg>');
}

Let me know if it works.

Let me know if it works.

Unfortunately, it didn't work — still incorrect padding and no checkmark

That's odd — the style works on my end in both v140 and Nightly v142.
Is it possible that something else is interfering?
Maybe worth confirming if the selector inside menupopup[needsgutter] properly excludes [checked="true"] using :not([image], [checked="true"]), so the checkmark isn't unintentionally hidden.

Maybe worth confirming if the selector inside menupopup[needsgutter] properly excludes [checked="true"] using :not([image], [checked="true"]), so the checkmark isn't unintentionally hidden.

Yep, that was my problem! Now it works, I updated the gist with corrected version of the selector.