The picture in picture is such a great feature. Thanks Firefox devs :)
I often had a window open only playing a youtube video, having it be native to the browser and always on top is such a great feature.
App developers spend a lot of their time making up for the poor decisions of OS developers. A good window manager would give you tabs for free, PiP (aka an always-on-top window) for free, dark mode for free. Instead OS devs go overboard in simplifying things to the point that app devs have to pay the cost of delivering what users want.
I'm stuck on a Mac, where I cannot always on top, hide the title bar, or alt-drag windows to move them.
With a proper PiP mode, that's no longer an issue.
On the other hand, Firefox keeps the close button for tabs on the right, even in macOS where every close button is on the left.
Thankfully, I've found a bit of userChrome that corrects this, as a workaround.