The latest version of Firefox 71.0 has picture-in-picture feature in video playback. It is available only on Windows and is expected to come in Mac and Linux versions next month.
Mozilla's new feature, the "image within image" as the company renders it in Greek, places a video in the lower right corner of your screen. The video can be played and you can surf or do any other work with the rest of the screen at the same time.
Video in Picture-in-Picture mode comes in front of each window and prevents other tabs or windows from hiding it.
Once you have downloaded the latest version of Firefox, hover over a video that is playing or pausing, and the blue Picture-in-Picture option will appear. Click the button and the video will open in a window, but you can move it according to your needs. The feature is only available in the Windows version, but it will come, according to Mozilla, in January and Mac and Linux versions.