UserChrome.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/tabs_on_bottom_v2.css made available under Mozilla Public License v. 2.0 See the above repository for updates as well as full license text. */

/* This reorders toolbar to place tabs below other toolbars. Requires Firefox 133+ */

@media (-moz-bool-pref: "userchrome.force-window-controls-on-left.enabled"){

 #nav-bar > .titlebar-buttonbox-container{
   order: -1 !important;
   > .titlebar-buttonbox{
     flex-direction: row-reverse;
   }
 }

} @media not (-moz-bool-pref: "sidebar.verticalTabs"){

 .global-notificationbox,
 #tab-notification-deck,
 #TabsToolbar{
   order: 1;
 }
 #TabsToolbar > :is(.titlebar-spacer,.titlebar-buttonbox-container){
   display: none;
 }
 :root[sizemode="fullscreen"] #nav-bar > .titlebar-buttonbox-container{
   display: flex !important;
 }
 :root[tabsintitlebar] #toolbar-menubar:not([autohide="false"]) ~ #nav-bar{
   > .titlebar-buttonbox-container{
     display: flex !important;
   }
   :root[sizemode="normal"] & {
     > .titlebar-spacer{
       display: flex !important;
     }
   }
   :root[sizemode="maximized"] & {
     > .titlebar-spacer[type="post-tabs"]{
       display: flex !important;
     }
     @media (-moz-bool-pref: "userchrome.force-window-controls-on-left.enabled"),
       (-moz-gtk-csd-reversed-placement),
       (-moz-platform: macos){
       > .titlebar-spacer[type="post-tabs"]{
         display: none !important;
       }
       > .titlebar-spacer[type="pre-tabs"]{
         display: flex !important;
       }
     }
   }
 }

}