Firefox has support for a lot of extensions but they removed most of the support now which is a shame. It allowed me to play YouTube on the background, a feature that make people actually pay YouTube premium music.
I wonder if there's a similar extension for kiwi? Basically it just nerds to disable the page visibility and full-screen API + some tweaks
just install "New pipe" from f-droid, it really rocks. Allows you to play videos in background, download video or audio - practically everything. The only issue is that it sometimes breaks when youtube changes its protocol