denser firefox vertical tabs

userChrome.css

.tabbrowser-tab {
    max-width: none !important;
    --tab-min-height: 0px;
    --inline-tab-padding: 0px;
    border: 1px solid var(--tab-selected-bgcolor) !important;
}

sidebar-main[expanded] div#vertical-tabs {
    width: 300px !important;
    max-width: none !important;
}

.tab-text {
    font-size: 11px;
}

.tab-label-container {
    height: 1.75em !important;
}

hmm seems the first

.tabbrowser-tab {
    max-width: none !important;

breaks closing tabs. It leaves a space where the closed tab used to be

a partial fix to the tabs not closing is

.tabbrowser-tab[fadein] {
    max-width: unset !important;
}

it's cause of some offending code in tabbrowser.js... this still causes a 3 second delay until the tabs actually close

a better fix is to disable animations with user_pref("ui.prefersReducedMotion", 1);