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 problem with firefox on android is that its slow af compared to chromium based alternatives. I can't see a difference on pc but it is clearly noticeable on android.
I also have this issue where the webview will refuse to change after a while.
I can try to leave a site typing in and address or try to switch tabs the UI will indicate the page changes and is loading but will still show the old site.
Eventually it goes black and is stuck until I force close and reload the page.
Haven't yet figured out how /where to report as a bug