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.
There are simpler ways - using newpipe, grayjay, etc.
It’s just a single install. But fair point. This allows you to create playlists, though.
Newpipe also let’s you make Playlist and subs.