Websites such as YouTube have taken the lead in supporting this feature. Just play any video, right-click it and select the "Paint in Paint" option to start it, or hover over the video, waiting for the function icon to appear on the right edge of the video window. Video will be played in a mini window at the bottom right of the screen. You can move to the right of the screen, resize and paste it where you want it on the screen.
PiP mode also provides function buttons to return the original video as well as pause/play controls. You can switch this function by selecting the options in the Video Context Menu. Firefox displays the message "This video is playing in Picture-in-Picture mode" in the main video window played in the Picture-in-Picture Player.
Note that Mozilla's Picture for Picture for Video is not compatible with Chrome's Picture-in-Picture Web API for developers. Although Mozilla initially showed interest in Chrome PiP, it has developed its own functionality.