fenix-fox/chrome/multi-row_tabs.css

/* Makes tabs to appear on multiple lines */
/* Tab reordering will not work and can't be made to work */
/* On Firefox 66 you can use multi-row_tabs_window_control_patch.css to hide window controls if you wish */

/* It's recommended to move tabs new-tab-button outside tabs toolbar */

/* Change the --multirow-n-rows to change maximum number of rows before the rows will start to scroll  */
/* Scrollbar can't be clicked but the rows can be scrolled with mouse */
/* This maximum visible rows won't work before Fx66 */
/* So this setting does nothing on Fx65 and all tab rows will be shown */
:root{ --multirow-n-rows: 3 }

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

/* Test for Firefox > 66 */
@supports (inset-block:auto){
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox > .arrowscrollbox-scrollbox{
    display: flex;
    flex-wrap: wrap;
    overflow-y: auto;
    max-height: calc(var(--tab-min-height) * var(--multirow-n-rows));
    scrollbar-color: var(--toolbar-bgcolor) var(--lwt-accent-color);
    scrollbar-width: thin;
  }
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox {
    overflow: -moz-hidden-unscrollable;
    display: block;
  }
}

/* Test for Firefox < 66 */
@supports not (inset-block:auto){
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox{
    min-height: unset !important;
  }
  #tabbrowser-tabs .scrollbox-innerbox{
    display: flex;
    flex-wrap: wrap;
  }
  #tabbrowser-tabs .arrowscrollbox-scrollbox {
    overflow: -moz-hidden-unscrollable;
    display: block;
  }
}

.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: 100px !important;
  flex-grow: 1;
  /*
  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% }

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