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.
But you don’t need an extension for that? Just standard firefox can already do that.
This might be about the auto pause that happens during playlist. However OP did not say so I might be wrong.