[ihuman]

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.

[snailmailman]

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.