for now.

given this trend of removing little usability and customization abilities here and there, one can only assume the custom CSS will be removed as well. especially seeing as its label has "legacy" right in it's name: toolkit.legacyUserProfileCustomizations.stylesheets

they'll give some reason like "only 1% of users use it" or, "it'll take too much time to make it work with our new, annual UI redesign/overhaul"

that being said, I'm sure someone (either on github or on reddit in /r/FirefoxCSS) will have the necessary custom CSS written to reproduce as soon as the version removing it hits the stable release