This addon works well on Chrome, but switching back to Firefox, it's finicky.

On Youtube, the addon only kicks in after going fullscreen and leaving fullscreen once.

On Twitch, the behavior is similar, but when clicked the third time does not pop up the menu, but goes into Twitch's "Theatre Mode" and opens the menu at the same time. On Chrome, the window goes into "Theatre Mode" and then pops up the menu on the first click.

I don't know if this is because the Firefox version is missing the "PiP" option?