You can modify parts of the Firefox chrome (i.e. parts that aren't the webpage) by modifying your userChrome.css file.

http://kb.mozillazine.org/index.php?title=UserChrome.css

Here's a few simple examples:

https://github.com/Timvde/UserChrome-Tweaks/tree/master/tabs

And a more thorough one:

https://old.reddit.com/r/FirefoxCSS/comments/7emhsq/my_compa...