I tried this too, but as Mozilla has no regard for any customization that are not done via the official graphical configuration interface (and sometimes not even that, considering how the compact UI mode is being removed), you'll have to find out how to fix it every time you update your browser.
I don't think I've ever updated my userChrome.css file, certainly not in the past few years. The browser chrome is pretty stable since this is how the browser itself is styled, so outside of big UI updates like Proton (which didn't even impact my own css) the chance of breakage is pretty slim.
