thunderbird-userchrome.css

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

@-moz-document url("chrome://browser/content/browser.xul") {
    :root:not([tabsintitlebar]) #navigator-toolbox {
        display: contents !important;
    }

    :root:not([tabsintitlebar]) #toolbar-menubar          { -moz-box-ordinal-group: 1 !important; }
    :root:not([tabsintitlebar]) #nav-bar                  { -moz-box-ordinal-group: 2 !important; }
    :root:not([tabsintitlebar]) #PersonalToolbar          { -moz-box-ordinal-group: 3 !important; }
    :root:not([tabsintitlebar]) #navigator-toolbox::after { -moz-box-ordinal-group: 4 !important; }
    :root:not([tabsintitlebar]) #content-deck             { -moz-box-ordinal-group: 5 !important; }
    :root:not([tabsintitlebar]) #TabsToolbar              { -moz-box-ordinal-group: 6 !important; }
} */

/* #TabsToolbar
{
  position: fixed;
  bottom: -0em;
  width: 100%;
}
#tabbrowser-tabs { width: 96%; }
#content-deck { margin-bottom: 0em; }

TabsToolbar:not([inFullscreen="true"]) {position:fixed; bottom:0em; width:100%;} */

#tabbrowser-tabs
{
  --tab-line-color: red !important;
}