Minimal firefox

userChrome.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/autohide_bookmarks_and_main_toolbars.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

#navigator-toolbox{
  --uc-bm-padding: 4px; /* Vertical padding to be applied to bookmarks */
  --uc-bm-height: calc(20px + 2 * var(--uc-bm-padding)); /* Might need to adjust if the toolbar has other buttons */
  --uc-navbar-height: -40px; /* navbar is main toolbar. Use negative value */
  --uc-autohide-toolbar-delay: 600ms; /* The toolbar is hidden after 0.6s */
}

:root[uidensity="compact"] #navigator-toolbox{
  --uc-bm-padding: 1px;
  --uc-navbar-height: -34px;
}
:root[uidensity="touch"] #navigator-toolbox{ --uc-bm-padding: 6px }

:root[sessionrestored] #nav-bar,
:root[sessionrestored] #PersonalToolbar{
  background-image: linear-gradient(var(--toolbar-bgcolor),var(--toolbar-bgcolor)), var(--lwt-additional-images,var(--toolbar-bgimage))  !important;
  background-position: top,var(--lwt-background-alignment);
  background-position-y: calc(0px - var(--tab-min-height) - 2*var(--tab-block-margin,0px));
  background-repeat: repeat,var(--lwt-background-tiling);
  transform: rotateX(90deg);
  transform-origin: top;
  transition: transform 135ms linear var(--uc-autohide-toolbar-delay) !important;
  z-index: 2;
}

:root[sessionrestored] #PersonalToolbar{
  z-index: 1;
  background-position-y: calc(0px - var(--tab-min-height) - 2*var(--tab-block-margin,0px) + var( --uc-navbar-height));
}

:root[lwtheme-image] #nav-bar,
:root[lwtheme-image] #PersonalToolbar{
  background-image: linear-gradient(var(--toolbar-bgcolor),var(--toolbar-bgcolor)),var(--lwt-header-image), var(--lwt-additional-images,var(--toolbar-bgimage)) !important;
}

#nav-bar[customizing],#PersonalToolbar[customizing]{ transform: none !important }

#navigator-toolbox > #PersonalToolbar{
  transform-origin: 0px var(--uc-navbar-height);
  position: relative;
}

:root[sessionrestored]:not([customizing]) #navigator-toolbox{
  margin-bottom:  calc(0px - var(--uc-bm-height) + var(--uc-navbar-height));
}
#PlacesToolbarItems > .bookmark-item {
  min-height: calc(var(--uc-bm-height) - 4px); /* Bookmarks have 2px block margin */
  padding-block: 0px !important;
}

#OtherBookmarks,
#PlacesChevron,
#PersonalToolbar > #import-button{
  padding-block: var(--uc-bm-padding) !important;
}

/* Make sure the bookmarks toolbar is never collapsed even if it is disabled */
:root[sizemode="fullscreen"] #PersonalToolbar,
#PersonalToolbar[collapsed="true"]{
  min-height: initial !important;
  max-height: initial !important;
  visibility: hidden !important
}
#PersonalToolbar[collapsed="true"] #PlacesToolbarItems > *,
:root[sizemode="fullscreen"] #PersonalToolbar #PlacesToolbarItems > *{
  visibility: hidden !important;
}

/* The invisible toolbox will overlap sidebar so we'll work around that here */
#navigator-toolbox{ pointer-events: none; border-bottom: none !important; }
#PersonalToolbar{ border-bottom: 1px solid var(--chrome-content-separator-color) }
#navigator-toolbox > *{ pointer-events: auto }

#sidebar-box{ position: relative }

/* Selected tab needs higher z-index now to "hide" the broder below it */
.tabbrowser-tab[selected]{ z-index: 3 !important; }

/* SELECT TOOLBAR BEHAVIOR */
/* Comment out or delete one of these to disable that behavior */

/* Show when urlbar is focused */
#nav-bar:focus-within + #PersonalToolbar,
#navigator-toolbox > #nav-bar:focus-within{
  transition-delay: 100ms !important;
  transform: rotateX(0);
}

/* Show when cursor is over the toolbar area */
#navigator-toolbox:hover > .browser-toolbar{
  transition-delay: 100ms !important;
  transform: rotateX(0);
}
/* This makes the tab notification box show immediately below tabs, otherwise it would break the layout */
#navigator-toolbox > div{ display: contents }
:where(#titlebar,#tab-notification-deck,.global-notificationbox){
  -moz-box-ordinal-group: 0;
}
/* Show when cursor is over popups/context-menus - cannot control which ones */
/*
#mainPopupSet:hover ~ box > toolbox > .browser-toolbar{
  transition-delay: 100ms !important;
  transform: rotateX(0);
}
*/

#navigator-toolbox-background {
  margin-top: -3px;
}

#alltabs-button {
  display: none;
}
/*
How To:
1. Find your firefox profile folder : https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data
2. create a "chrome" folder if it doesn't exist
3. Paste content of this gist in a "userChrome.css" file
4. Go to about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true
5. Restart Firefox

Don't hesitate to say if there are bugs, I only tested this for my workflow : I don't use tabs
at all but I want to see the bar if I open a tab by mistake (otherwise I sometimes lose a tab)
*/

/* Copy the default settings for --tab-min-height as --hidetabs-tab-min-height */
:root {
    --hidetabs-tab-min-height: 33px;
}

:root[uidensity=compact] {
    --hidetabs-tab-min-height: 29px;
}

:root[uidensity=touch] {
    --hidetabs-tab-min-height: 41px;
}

/* Set --tab-min-height to 0px so tab bar can disappear */
#tabbrowser-tabs {
    --tab-min-height: 0px;
}

/* Restore minimum height when more than one tab */
#tabbrowser-tabs tab {
    min-height: var(--hidetabs-tab-min-height);
}

/* Collapse tab bar when there is only one tab (tab is both first & last) */
#tabbrowser-tabs tab[first-visible-tab="true"][last-visible-tab="true"] {
    visibility: collapse;
}

/* Hide the New Tab button when there is only one tab (first visible tab is
   adjacent to new tab button) */
#tabbrowser-tabs tab[first-visible-tab="true"] + #tabs-newtab-button {
    visibility: collapse;
}