Mozilla Firefox has introduced an experimental Picture-in-Picture (PiP) feature in its @FirefoxNightly build, which activates when a video-playing tab is moved to the background. This can be enabled via about:settings#experimental. The feature will be included in Firefox version 130.
https://alternativeto.net/news/2024/8/firefox-is-introducing-an-exciting-automatic-picture-in-picture-mode-for-background-tabs/
@alternativeto @FirefoxNightly given the number of websites that start playing videos on page load, that sounds terrible