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.
Background playback used to be a thing in the official app, but then they obviously couldn’t get as many people to pay for YouTube Music or YouTube Premium or whatever.