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.
RiMusic is update regularly, no third party player is perfect, even PipiPipe or Newpipe can break suddenly, Google/YouTube don’t like third party apps using their services and are always implementing ways to break apps