I asked this exact question on Stack Overflow earlier today and found that someone else had already also asked it and got an answer.

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.

What uglycoyote commented is the stopgap solution for now, but an API for doing that is in the works, so in a few versions from now, you shouldn't need that more-or-less hack anymore.