Out of the box it can play audio in the background, and now that extensions are available you can block ads as well. You don’t really need constantly updating 3rd party clients or questionable firewalls anymore if you just want usable youtube on a phone.
The only issue I have is sometimes when I try to fullscreen a video, Firefox then tries to pop it out and then kind of crashes? It’s more like returns to homescreen, the video is playing, but not visible, and if I reopen or click over to Firefox it’s just the top right corner of Firefox in the bottom left corner of my screen. I’ve tried everything I can think of to fix it.