Firefox 113 has added the ability to watch a video from a website anywhere on your monitor, as long as you stay on the web page so you can keep on using.
Useful if you are trying to do something else at the same time, such as look at other web tabs.

Just play the video, press the picture-in-picture icon which looks like an arrow in a big box pointing at a small box. Or press Ctrl-shift-]. Then drag the video anywhere you like.
By the way, if you are still using Windows 7, 8 or 8.1 then Firefox support ends in Sept 2024. Most others have already ended support. Think about upgrading to Windows 10.