userChrome.css

/* 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{ --uc-window-control-width: 0px !important }

#navigator-toolbox{ padding-top: calc(1px + var(--uc-titlebar-padding,0px)) !important }

#toolbar-menubar{
position: fixed;
display: flex;
top: var(--uc-titlebar-padding,29px);
height: 29px;
width: 100%;
overflow: hidden;
}

#nav-bar-customization-target,
#PanelUI-button,
#nav-bar-overflow-button,
#toolbar-menubar > spacer{ flex-grow: 1 }
#toolbar-menubar > spacer,
#toolbar-menubar > .titlebar-buttonbox-container{ order: 1000 }


#toolbar-menubar:not([customizing]){
  --uc-menubar-height: 34px;
  position: fixed;
  display: flex;
  width: 100vw;
  height: var(--uc-menubar-height);
  margin-top: calc(8px - var(--uc-menubar-height));
  transition: margin-top 150ms linear !important;
  -moz-window-dragging: drag;
  z-index:4;
}

#toolbar-menubar:hover{
  background-color: var(--toolbar-bgcolor) !important;
  margin-top: 0px;
  z-index: 6
}

/* This exists only for compatibility reasons with some other styles */
#navigator-toolbox{ --uc-hide-window-control-space: 0 }


/* TABS: height */*|*:root { --tab-toolbar-navbar-overlap: 0px !important; --tab-min-height: 25px !important;
--tab-min-width: 80px !important;

#tabbrowser-tabs {
width: 100vw !important;
}
#main-window:not([chromehidden*="toolbar"]) #navigator-toolbox {padding-bottom: var(--tab-min-height) !important;}

.tab-background {
border-radius: 8px 8px 0px 0px !important; border-image: none !important;
}
.tab-line {
display: none;
}

.tab-close-button {
color: red!important;
}