Earlier quoted context omitted.
That would be an excellent way to force Google to switch distribution of all content via YouTube to HTML5 EME with Widevine DRM, given it's pretty clear their music agreements prohibit permitting this.
That’s their problem. A browser should always give a user the upper hand. Browser block popups, forcefully enable right-click on sites that disable it, browsers allow users to forcefully enable zoom, or background playback. And just like that, browsers should also allow users to forcefully enable PiP.
Too bad Mozilla won't ever dare to defend themselves from Google's bullying and will happily accept having background playback on YouTube, until now one of the primary motives for people to try Fennec out, being taken away.
https://bugzilla.mozilla.org/show_bug.cgi?id=1355407
RESOLVED WONTFIX