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.
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Also, it sucks the battery empty while running in the background (more than chrome-based browsers).