A fresh test build of Firefox Nightly 86 supports multiple picture-in-picture windows.
Most modern browsers have the function of bringing video to a separate window. It is displayed on top of other applications. This feature helps you work on your computer and watch videos at the same time. For example, this is convenient when you are watching tutorial videos.
In the new Firefox Nightly 86, you can open multiple video tabs and enable Picture-in-Picture for each. By default, all videos are played simultaneously, just like audio. All of the controls for Picture-in-Picture mode are present in the browser and can be customized.