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.
Thanks! I’ve been struggling with this for months, couldn’t even find someone with the same issue.
For anyone else with this problem: Go to Android settings (not Firefox settings), then apps, Firefox, and turn off picture-in-picture
I’m glad it worked for you!