Sadly it's less than useful on MacOS (Still not stable, you have to manually turn it on in about:config). You cannot resize it, and it doesn't stay on top.

It does all of that on Nightly. Will get to release in a couple of months.

It's an interesting. In Chrome the way you can drag tabs out of the window and easily arrange youtube-tabs on the screen while they play works much smoother than in FF. But this new FF approach is kind of another way of solving that problem.

Can you make them stay on top in Chrome?

I want to try it, but it’s windows-only right now I think

It is, so they say. They said it should be coming to macOS and Linux in Janurary 2020.

https://blog.mozilla.org/blog/2019/12/03/news-from-firefox-o...

Its in 71 on macOS (and maybe Linux), but its disabled by default. In about:config, search for picture-in-picture, and enable everything except for always-show (unless you want that). If you right-click a video, there will be a new picture-in-picture option. For Youtube, you need to double-right-click to bring up Firefox's right-click menu.

Can confirm that these options exist and appear to work in linux, at least for me. (ubuntu 16.04) And im still on v70, so i guess its been there for a little while at least.

Working on Ubuntu for me.