Tree-style tabs won't be around much longer.
https://superuser.com/questions/1261660/firefox-quantum-ver-...
(basically, hacking some CSS rules by creating a file in your AppData directory, so that the top bar is hidden)
Incidentally, I also discovered that Tree Style Tab's settings page under Firefox has a nice little box where you can configure the CSS it uses, which was nice because I prefer it with a smaller font and less padding.