In my case, I need the popped out window to stay on top.

Edit: There is no way to bring back the popped up window to your tabs. I suggest to have a button on top right corner.

I use AutoHotKey to fix Stay on top issue:
https://www.howtogeek.com/196958/the-3-best-ways-to-make-a-window-always-on-top-on-windows/

https://www.howtogeek.com/196958/the-3-best-ways-to-make-a-window-always-on-top-on-windows/

I use this add-on to open videos and play them while browsing. I know there is Popup Video(WebExtension), but here is why I don't use it:
1- the video does not resume from where it was.
2- you can't go back to the tab and resume from where you stopped (currently I have the same issue with Popup Window too, but I hope that would be fixed.
3- it is not seamless, but i assume that is Mozilla fault too, though, I think if Popup video worked like popup window it would've been more seamless and can support more site. I zoom out to make the window smaller for Facebook videos, but that ruin my other Facebook tabs, so take that in mind.

I used the similar feature on Opera. Try it and see how amazing it is. Even when you popped out a video you will see it's playing on the main tab and on the popped video simultaneously, which I assume it is basically mirroring the video portion. it is seamlessly fast.

to sum it, popup window is better than Popup Video(WebExtension), and of course you need AutoHotKey for both of them, otherwise, it is pointless. oh, Popup Video(webExtension) is much much better than Min Vid... I think

Thanks for your hard work <3