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.
The fact that they attempted them at all is a breach of trust (and I say that as someone who’s been a Firefox user and advocate without interruption since all the way back when it was called “Phoenix”).