https://www.reddit.com/r/FirefoxCSS/
A whole sub dedicated to making firefox look however you want it to. You can edit the entire browser with css like a website.
And each update you have to adjust your CSS to fix whatever the UI changes broke. That is of course until Mozilla remove userStyle.css entirely - they have already disabled it by default so for new installs you have to toggle toolkit.legacyUserProfileCustomizations.stylesheets in about:config.