You can still tweak Firefox internals but you have to resort to its native `userChrome.css` [0] since current extensions API does not allow access to every corner. Luckily you can use devtools for that [1] what makes it quite pleasant (at least you don't have to restart browser to see the change). You can use pre-made package ([2][3]) and toggle what you want.

[0] https://www.userchrome.org/ [1] https://www.reddit.com/r/FirefoxCSS/comments/73dvty/tutorial... [2] https://github.com/Aris-t2/CustomCSSforFx/ [3] https://github.com/Timvde/UserChrome-Tweaks