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.
You know Firefox for android does this natively. No plugin required.
That was my thought… I was thinking “I already do this without an extension”, so I had to verify that I wasn’t using an extension that I forgot about.