For a serious answer, here's some links:

https://w3c.github.io/picture-in-picture/

https://github.com/w3c/picture-in-picture/blob/master/explai...

https://developers.google.com/web/updates/2018/10/watch-vide...

https://bugzilla.mozilla.org/show_bug.cgi?id=1463402

https://www.chromestatus.com/feature/5729206566649856

Looks to me like it is a standard that web devs requested (e.g. so you can scroll down YouTube or whatever and the video will keep playing in the corner), which Firefox chose to also make an end-user UI for.