For a serious answer, here's some links: https://w3c.github.io/picture-in-picture/ https://w3c.github.io/picture-in-picture/ https://github.com/w3c/picture-in

For a serious answer, here's some links: https://w3c.github.io/picture-in-picture/ https://w3c.github.io/picture-in-picture/ https://github.com/w3c/picture-in-picture/blob/master/explainer.md https://github.com/w3c/picture-in-picture/blob/master/explai... https://developers.google.com/web/updates/2018/10/watch-video-using-picture-in-picture https://developers.google.com/web/updates/2018/10/watch-vide... https://bugzilla.mozilla.org/show_bug.cgi?id=1463402 https://bugzilla.mozilla.org/show_bug.cgi?id=1463402 https://www.chromestatus.com/feature/5729206566649856 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.