this should work

README.md

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+uses files from https://github.com/MrOtherGuy/firefox-csshacks

imports/tabs_fill_available_width.css

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+/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/tabs_fill_available_width.css made available under Mozilla Public License v. 2.0
+See the above repository for updates as well as full license text. */
+
+/* Why 100vw? Tab closing requires width animation to end and "none" can't be animated */
+.tabbrowser-tab[fadein]{ max-width: 100vw !important }

imports/tabs_on_bottom.css

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+/* 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-win7),(-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 }

userChrome.css

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 @import url("tabs_on_bottom.css");
 @import url("tabs_fill_available_width");
-@import url("blank_page_background");
-@import url("centered_tab_content");
 .tabbrowser-tab[fadein]:not([pinned]) { 
 	max-width: none !important; 
 }