You can actually play YouTube in the background with Firefox on Android. There is two ways, 1. Put the video in full screen mode and then press the system home button, this enabled PiP. 2. Start the video, click onto another tab in Firefox (this will pause the video) but then with that second tab active, open the tab switcher and press the play button beside the other tab with the video. Then it will play in the background until you interact with the tab again.