#hm... annoyed me since long... found a solution to reduce #height of #tab #bar...

add this #css #code (the hashtags have to be in the code, they are not for D*)

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

/* TABS: height /
:root {
--tab-toolbar-navbar-overlap: 0px !important; / fix for full screen mode */
--tab-min-height: 25px !important;
}
:root #tabbrowser-tabs {
--tab-min-height: 25px !important;
}
#TabsToolbar {height: var(--tab-min-height) !important;}

from #Firefox #support https://ishort.ink/t6ZJ