I used to be able to do this in Vanilla Firefox, even when my phone was in sleep mode, but lately it stopped working.

Van confirm that this extension work on allowing video to play in the background, from my few test (been using it for around 20 minutes)

Good

Does not work with YT Music🤔

Работает в мобильной версии Firefox. Воспроизведение в фоне, и при переходе между вкладками - работает. Спасибо 👍🤙