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.
To top up your comment, you can get the permissions required for each extension in Firefox for Android. Extension Manager --> click the extension --> Permissions
In the case of this specific extension, it requires access to the data of only 3 domains :
Many other extensions are far worst in regards to permissions that this one.
It is a good habit to check what are the permissions required before using any extension or phone apps for that matter.
Thanks for the reminder!