This is normally a window manager feature, why put it in browser where it's limited to browser tabs?
They implemented it in Firefox because Firefox is the one detecting and playing the video? MacOS already has a similar feature, but AFAIK it only works in Safari.
I'm curious how the WM could detect that the a portion of a window is playing video, and break that out into it's own movable window?
I'm not sure how you do that without the browser managing that.
I can then interact with that video separately from the page. I can scroll around on the page, I can open other links. It decouples the video from the page.
There's lots of ways to open new chromeless window, or set a window always-on-top. That totally makes sense to be in a wm, but that's not all this is doing, right?