CSS FF 89

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

/* Modify to change window drag space width */
/*
Use tabs_on_bottom_menubar_on_top_patch.css if you
have menubar permanently enabled and want it on top
 */

/* IMPORTANT */
/*
Get window_control_placeholder_support.css
Window controls will be all wrong without it.
Additionally on Linux, you may need to get:
linux_gtk_window_control_patch.css
*/

:root{ --uc-titlebar-padding: 0px; }
@media (-moz-os-version: windows-win10){
  :root[sizemode="maximized"][tabsintitlebar]{ --uc-titlebar-padding: 8px }
}
#toolbar-menubar[autohide="true"] > .titlebar-buttonbox-container,
#TabsToolbar > .titlebar-buttonbox-container{
  position: fixed;
  display: block;
  top: var(--uc-titlebar-padding,0px);
  right:0;
  height: 40px;
}
/* Mac specific. You should set that font-smoothing pref to true if you are on any platform where window controls are on left */
@supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled"){
  :root{ --uc-titlebar-padding: 0px !important }
  .titlebar-buttonbox-container{ left:0; right: unset !important; }
}

:root[uidensity="compact"] #TabsToolbar > .titlebar-buttonbox-container{ height: 32px }

#toolbar-menubar[inactive] > .titlebar-buttonbox-container{ opacity: 0 }

#navigator-toolbox{ padding-top: var(--uc-titlebar-padding,0px) !important; }

.titlebar-buttonbox-container > .titlebar-buttonbox{ height: 100%; }

#titlebar{
  -moz-box-ordinal-group: 2;
  -moz-appearance: none !important;
  --tabs-navbar-shadow-size: 0px;
}

.titlebar-placeholder,
#TabsToolbar .titlebar-spacer{ display: none; }
/* Also hide the toolbox bottom border which isn't at bottom with this setup */
#navigator-toolbox::after{ display: none !important; }

@media (-moz-gtk-csd-close-button){ .titlebar-button{ -moz-box-orient: vertical } }

/* These exist only for compatibility with autohide-tabstoolbar.css */
toolbox#navigator-toolbox > toolbar#nav-bar.browser-toolbar{ animation: none; }
#navigator-toolbox:hover #TabsToolbar{ animation: slidein ease-out 48ms 1 }

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

/* Menubar on top patch - use with tabs_on_bottom.css */
/* Only really useful if menubar is ALWAYS visible */

:root:not([sizemode="fullscreen"]){ --uc-window-control-width: 0px !important }

#navigator-toolbox{ padding-top: calc(29px + var(--uc-titlebar-padding,0px)) !important }
:root[sizemode="fullscreen"] #navigator-toolbox{ padding-top: 0px !important; }
#toolbar-menubar{
  position: fixed;
  display: flex;
  top: var(--uc-titlebar-padding,0px);
  height: 29px;
  width: 100%;
  overflow: hidden;
}

#toolbar-menubar > .titlebar-buttonbox-container{ height: 29px; order: 100; }

#toolbar-menubar > [flex]{ flex-grow: 100; }
#toolbar-menubar > spacer[flex]{
  order: 99;
  flex-grow: 1;
  min-width: var(--uc-window-drag-space-width,20px);
}

#toolbar-menubar .titlebar-button{ padding: 2px 17px !important;  }

#toolbar-menubar .toolbarbutton-1 { --toolbarbutton-inner-padding: 3px }