This is definitely within my skillset but far from user-friendly. I'm just remarking on how simple it is to put a toggle in the settings to show/hide the tab bar and they haven't done this because they're pushing a particular look that is very similar to the design choices for Chrome.
It could be easier, but still doable in 5 minutes (that's how long it took me) with some CSS, see https://github.com/piroor/treestyletab/wiki/Code-snippets-fo... on how to do it.
There seem to be some security concern or it's not so easy UX wise, see https://bugzilla.mozilla.org/show_bug.cgi?id=1332447 for all the discussions and progress.