The only remaining quirk with TST I have noticed is that sometimes on launch, tab groups open expanded even though I had them collapsed. Then I have to first collapse any child groups before it agrees to collapse the whole thing.
Wow that is a lot of Github commits. The last release is about a month ago... Have you ever tried using the bleeding edge code? Looking at https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Yo... it seems like it might be possible, but you'd have to manually 'install' the extension every time you start Firefox...
piroor is putting in a ton of work, see: https://github.com/piroor/treestyletab/commits/master
edit: oh, I've just ignored the tab strip at the top, so no comments on the tab hiding :)