Firefox 95 ultra-compact multirow tabs on top, context line below tab & compact menus - urbanhusky's ugly hack YMMV :)

userChrome.css

/* Hi, urbanhusky here.
   This is cobbled together from various sources. 
   One of those sources is: https://github.com/MrOtherGuy/firefox-csshacks
*/

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

/* Makes tabs to appear on multiple lines */
/* Tab reordering will not work and can't be made to work */
/* You can use multi-row_tabs_window_control_patch.css to move window controls to nav-bar*/

/* You might want to move tabs-new-tab-button outside tabs toolbar for smoother behavior */

/* Change the --multirow-n-rows to change maximum number of rows before the rows will start to scroll  */

:root{
    --multirow-n-rows: 5;
    --multirow-tab-min-width: 76px;
    --multirow-tab-dynamic-width: 0; /* Change to 0 for fixed-width tabs using the above width. */
		
		/* urbanhusky hacks */
		--inline-tab-padding: 2px; /* less space wasted left/right inside tab */
		--tab-label-mask-size: 1em; /* less fading */
}

/* Scrollbar can't be clicked but the rows can be scrolled with mouse wheel */
/* Uncomment the next line if you want to be able to use the scrollbar with mouse clicks */

/* #tabbrowser-arrowscrollbox{ -moz-window-dragging: no-drag } */

/* Uncommenting the above makes you unable to drag the window from empty space in the tab strip but normal draggable spaces will continue to work */

#tabbrowser-tabs{
  min-height: unset !important;
  padding-inline-start: 0px !important
}

/* urbanhusky hack: make tabs *much* more compact */
.tabbrowser-tab{ 
	height: var(--tab-min-height); 
	/* border-top: 0; /* remove top-border on tabs (causes grey line on dark theme */
}

/* uh: prevent scrollbar on multirow tabs with smaller tabs */
#TabsToolbar {
	--toolbarbutton-inner-padding: calc((var(--tab-min-height) - 8px) / 2) !important; /* add a little space to prevent scrollbar when not yet needed */
}

/* Adjust tab corner shape, optionally remove space below tabs */

#tabbrowser-tabs {
    --user-tab-rounding: 4px;
}

/* Move container color bar to bottom of tab */
.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
    margin-top: calc(var(--tab-min-height) - 2px) !important;
}

/*** Tighten up drop-down/context/popup menu spacing (8 Sep 2021) ***/

menupopup:not(.in-menulist) > menuitem, 
menupopup:not(.in-menulist) > menu {
  padding-block: 4px !important; /* reduce to 3px, 2px, 1px or 0px as needed */ 
  min-height: unset !important; /* v92.0 - for padding below 4px */
}
:root {
  --arrowpanel-menuitem-padding: 4px 8px !important;
}

@-moz-document url(chrome://browser/content/browser.xhtml){
  
  #scrollbutton-up~spacer,
  #scrollbutton-up,
  #scrollbutton-down{ display: var(--scrollbutton-display-model,initial) }

  scrollbox[part][orient="horizontal"]{
    display: flex;
    flex-wrap: wrap;
    overflow-y: auto;
    max-height: calc((var(--tab-min-height) + 2 * var(--tab-block-margin,0px)) * var(--multirow-n-rows));
    scrollbar-color: currentColor transparent;
    scrollbar-width: thin;
    scroll-snap-type: y mandatory;
  }
}

.scrollbox-clip[orient="horizontal"],
#tabbrowser-arrowscrollbox{
  overflow: -moz-hidden-unscrollable;
  display: block;
  --scrollbutton-display-model: none;
}

.tabbrowser-tab{ scroll-snap-align: start; }

#tabbrowser-tabs .tabbrowser-tab[pinned]{
  position: static !important;
  margin-inline-start: 0px !important;
}

.tabbrowser-tab[fadein]:not([pinned]){
  min-width: var(--multirow-tab-min-width) !important;
  flex-grow: var(--multirow-tab-dynamic-width);
  /*
  Uncomment to enable full-width tabs, also makes tab dragging a tiny bit more sensible
  Don't set to none or you'll see errors in console when closing tabs
  */
  /*max-width: 100vw !important;*/
}

.tabbrowser-tab > stack{ width: 100%; height: 100% }

/* remove bottom margin so it doesn't throw off row height computation */
#tabs-newtab-button{ margin-bottom: 0 !important; }

#alltabs-button,
:root:not([customizing]) #TabsToolbar #new-tab-button,
#tabbrowser-arrowscrollbox > spacer,
.tabbrowser-tab::after{ display: none !important }


/*** Audio Playing / Mute Button when sound is playing (urbanhusky hack: without this the button does not even show) ***/

#TabsToolbar {
    --user-mute-button-height: 16px;  /* default size is 12px, site icon is 16px */
}

/* enlarge it */
.tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]) {
    width: var(--user-mute-button-height) !important;
    height: var(--user-mute-button-height) !important;
    /* margin-left: calc(var(--user-mute-button-height) / 2 + 2px) !important; /* pushes icon to the right */
    /* margin-right: 2px !important; /* closes up some space before the text */
    padding: 0 !important; /* allows icon to expand to full size */
}

/* Color the icon on hover for confirmation or avoidance */
.tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]):hover {
    fill: green !important;
}
[lwthemetextcolor="bright"] .tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]):hover {
    fill: lightgreen !important; /* for dark themes */
}