Hi.

You can move the CSS code for tabs on bottom that works in Firefox71 to its own file (e.g. tabs-on-bottom.css). Do NOT place the default @namespace line at the top of this tabs-on-bottom.css file. Add a line above the @namespace line in userChrome.css to import this file. If you do not have other CSS code in userChrome.css then you can consider to keep the code in userChrome.css and only remove the @namespace line.

See also:

• https://support.mozilla.org/en-US/kb/forum-response-tabs-below-url-bar

You need to remove this line in Firefox 108 and newer.

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

userChrome.css:

@import url("tabs-on-bottom.css");
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

Place this code in tabs-on-bottom.css code updated for for Firefox 108+.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* TABS: below navbar - Firefox 65-89+,95+,106+,108+ */

/* https://searchfox.org/mozilla-release/source/browser/themes/shared/tabs.inc.css */
/* https://github.com/Aris-t2/CustomCSSforFx/blob/master/classic/css/tabs/tabs_below_navigation_toolbar_fx89.css */

/* ROOT: VARS */
*|*:root {
  --tab-toolbar-navbar-overlap: 0px !important;

  --tab-min-height: 26px !important;
  --tab-min-width:  60px !important; /*minimum:50px*/
/*  --tab-overflow-pinned-tabs-width: 175px */

  --tabbar-adjust:  0px;
  --tabbar-caption: 0px;

  --tabbar-caption-padding:  90px;
  --tabbar-controls-padding: 90px;
}

/* TABS: below nav-bar */
#navigator-toolbox toolbar:not(#nav-bar):not(#toolbar-menubar) {-moz-box-ordinal-group: 10 !important;}
#TabsToolbar {-moz-box-ordinal-group: 1000 !important;}

/* TABS: POSITION */
#TabsToolbar {
  display: block !important;
  position: absolute !important;
  bottom: 0;
  width: 100vw !important;
}

/* Firefox 107 and older *//* #tabbrowser-tabs { width: 100vw !important; } */

/* navigator-toolbox: PADDING */
*|*:root:not([chromehidden*="toolbar"]) #navigator-toolbox {
  position: relative !important; /*89*/
  padding-bottom: calc(var(--tab-min-height) + 1px) !important; /*ADJUST*/
  background-color: var(--toolbar-bgcolor) !important;
}

/* TABS: menubar,titlebar hidden - 65-73 *//*
*|*:root[tabsintitlebar]:not([inFullscreen="true"]):not([sizemode="maximized"])
 #toolbar-menubar[autohide="true"] ~ #TabsToolbar{
  bottom: var(--tabbar-adjust) !important; /*ADJUST*/
}
*/

/* TABS: HEIGHT */
#tabbrowser-tabs,
#xtabbrowser-arrowscrollbox,
#xtabbrowser-tabs .tabbrowser-tab {
  min-height: var(--tab-min-height) !important;
  max-height: var(--tab-min-height) !important;
}

#tabbrowser-tabs,
.tab-stack,
.tab-content {
  height: var(--tab-min-height) !important;
}

/* TABS: APPEARANCE */
#TabsToolbar {
  height: var(--tab-min-height) !important;
  margin-bottom: 1px !important;
  box-shadow: ThreeDShadow 0 -1px inset, -moz-dialog 0 1px !important; /*OPTIONAL*/
  background-color: var(--toolbar-bgcolor) !important;
  color: var(--toolbar-color) !important;
}

/* DRAG SPACE */
.titlebar-spacer[type="pre-tabs"],
.titlebar-spacer[type="post-tabs"] {
  width: 20px !important;
}

/* TABS: movingtab - vertical shifts */
#navigator-toolbox[movingtab] > #titlebar > #TabsToolbar {
  padding-bottom: unset !important;
}

#navigator-toolbox[movingtab] #tabbrowser-tabs {
  padding-bottom: unset !important;
  margin-bottom:  unset !important;
}

#navigator-toolbox[movingtab] > #nav-bar {
  margin-top: unset !important;
}

/* INDICATORS - POSITION */
*|*:root[privatebrowsingmode=temporary] .private-browsing-indicator {
  display: block !important;
  position: absolute !important;
  bottom: 0px !important;
  right: 0px !important;
  width: 14px !important;
  pointer-events: none !important;
}

/* INDICATORS - HIDE */
*|*:root:not([accessibilitymode])             .accessibility-indicator    {display: none !important}
*|*:root:not([privatebrowsingmode=temporary]) .private-browsing-indicator {display: none !important}

If you want to have the caption buttons visible on the Tab bar with the Menu bar and Title bar hidden then use this code as a start to replace the above posted "caption buttons - hide" rule. The caption button are positioned absolutely via a top property, so this is affected by the Bookmarks Toolbar (PersonalToolbar) being visible or hidden. Use either of the two "--personal-toolbar-height" settings that apply to the state (collapsed/hidden) of the Bookmarks Toolbar.

/* caption buttons - PersonalToolbar hidden/visible */
*|*:root {
 --personal-toolbar-height: 0px !important;   /*PersonalToolbar hidden*/
 --personal-toolbar-height: 24px !important;  /*PersonalToolbar visible*/
 --caption-buttons-adjust: -12px !important;  /*caption buttons - vertical adjustment*/
}

*|*:root[tabsintitlebar]:not([inFullscreen="true"]) #toolbar-menubar[autohide="true"] ~
 #TabsToolbar .titlebar-buttonbox-container {
  position: fixed !important;
  display: block !important;
  right: 0 !important;
  top: calc(var(--tab-min-height) + var(--personal-toolbar-height) + var(--caption-buttons-adjust));
  visibility: visible !important;
  height: var(--tab-min-height) !important;
}

/* caption buttons - HEIGHT and WIDTH */
.titlebar-buttonbox-container toolbarbutton {
  height: var(--tab-min-height) !important;
  margin: 0px -5px 0px -5px !important;
  padding: 0 !important;
}

/* caption buttons - PADDING - reserve space on Tab bar */
*|*:root[tabsintitlebar]:not([inFullscreen="true"])
 #toolbar-menubar[autohide="true"] ~
  #TabsToolbar {
   padding-right: 100px !important; /*adjust*/
}