/* 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(29px + var(--uc-titlebar-padding,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 }

/* 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;
}

Hey I found it, you need to do the whole chrome folder and about:config userprof thing and use this code in the css <pre><nowiki> /* 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(29px + var(--uc-titlebar-padding,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 } /* 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; }</nowiki></pre><br>

https://www.reddit.com/r/FirefoxCSS/comments/nq2d0q/tabs_on_bottom_for_firefox_89/ reddit thread I got the code from

#tabbrowser-tabs { width: 100vw !important; }

Note that in 108+ versions you need to remove this rule. #tabbrowser-tabs { width: 100vw !important; }