The F key is your friend. It puts the video full screen. You don't even have to find the full screen icon at the bottom right of the video, just hit F.
First to trigger the Javascript context menu, and again to trigger the native one which will have the "Picture in picture" option. And there are actually two different native context menus, so if the PiP one doesn't pop up, you might have to try multiple times.
Very weird UX instead of just giving us a PiP button under the video.
then, when you press `F` on a video, you will remove all firefox 'decoration', just like fullscreen mode, but it'll respect whatever position and size you have set for the browser.
full-screen-api.transition-duration.enter
full-screen-api.transition-duration.leave
full-screen-api.transition-duration.timeout
full-screen-api.warning.delay
full-screen-api.warning.timeout
Any website with a video player should include a "full window" control