Mozilla Firefox is adding a Picture-in-Picture Mode that allows users to pop the video out of the web page in a stay on top frame that they can watch while using other applications. The feature has been enabled in the Firefox Nightly build. To enable the feature, go to the about:config page and set the media.videocontrols.enabled setting to true using the following instructions. You can now go to YouTube and test this new feature while watching one of the videos.
Source: https://www.bleepingcomputer.com/news/software/mozilla-adding-a-picture-in-picture-mode-to-firefox/