Unfortunately userchrome.css breaks frequently with firefox updates and is a pain to maintain since it is outside of the extension ecosystem.

There used to be a great addon called ClassicThemeRestorer that got killed during firefox's terrible web extension debacle. It lives on in https://github.com/Aris-t2/CustomCSSforFx but different parts break everytime the fickle designers at mozilla change their mind on what a tab should look like.