It can be a bit buggy at times, but when it comes to just playing music with the screen locked, it’s perfect.
https://addons.mozilla.org/en-US/firefox/addon/video-background-play-fix/
Edit: I forgot to also mention that ad blocker extensions make sailing the seas on mobile much more manageable. Extentions, in general, have been a really nice addition.
Revanced.app and the GitHub the other person linked to are the only official sources.
An easy way to tell if you’re using the legit version is if you’re using ReVanced Manager to build the APK yourself. (Don’t worry, it’s easy and straightforward.) If you downloaded an APK directly, you have a fake copy.