you can style firefox with arbitrary css code!

userChrome.css (+also userContent.css, user.js if you need them)

just enable toolkit.legacyUserProfileCustomizations.stylesheets in the about:config page
then open your Firefox profile folder, create a folder called “chrome” and create two files: userChrome.css and userContent.css

you can style browser window elements using the userChrome.css

userContent.css applies to all pages, including your firefox home page which is usually impossible to style eith extensions for security purposes (use -moz-* queries to restrict to certain pages)