Firefox – Multi Row Tab Bar

Here's an updated userChrome.css that shows icons and hides the ugly scrollbar in the tab rows

UPDATE: Newer Firefox versions changed userChrome behavior again, now I've taken the script directly from here (with a minor change to the max rows and tab width): https://github.com/MrOtherGuy/firefox-csshacks/blob/master/chrome/multi-row_tabs.css:

/* 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: 6;
    --multirow-tab-min-width: 150px;
    --multirow-tab-dynamic-width: 1; /* Change to 0 for fixed-width tabs using the above width. */
}

/* 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
}

@-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) * var(--multirow-n-rows));
    scrollbar-color: currentColor transparent;
    scrollbar-width: thin;
  }
}

#tabbrowser-arrowscrollbox{
  overflow: -moz-hidden-unscrollable;
  display: block;
  --scrollbutton-display-model: none;
}

.tabbrowser-tab{ height: var(--tab-min-height); }

#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% }

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

Update July 7th 2020: UserChrome behavior has changed once again for FF78+, please update to the above or check the Github page for source CSS.