This add-on does exactly what I was looking for. It always annoys me that Youtubes miniplayer disappears when switching to a new tab. This popout player solves that problem and does a great job!

Works well, except on videos where the video ID is not in the URL itself (i.e. live streams). Example: https://www.youtube.com/c/euronews/live

Really great implementation, 5 stars if you can find a way to also ship it with "always on top" functionality. ;) But with five minutes of work on the individual's part, they could easily implement that themselves. Look up autohotkey and always on top in the same search folks. I'm not even sure add-ons for firefox can do something like that my self, considering that's more to do with your operating system than your web browser, lol.

to make it really good the poput player has to be on top of the regular firefox page or in general on the top of everything so that you could use it while watching some other content in the background

Works as expected.

Makes a new windows that plays the video but doesn't stay on top like I wish it could. also the title bar at the top is sorta annoying too :/

Amazingly Integrated! Uses built-in YouTube features like auto-play and start times, but with a very subtle button that fits right in with YouTube's own.
My only con is it has the URL and Firefox menu on it, which can disturb the experience, but I'm not sure that can be changed.