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.
Any of these (YouTube Music) players are better choices than using an extension on a browser.
Also better features like lyrics, better interface…
I’m pretty sure extensions can request browsing data for some specific URLs rather than every site you visit. If an extension with functionality on one website asks for access to all of them, then yeah it’s a bad sign.