Some additional debugging info:
With two screens, open two fullscreen browser videos using firefox.
Pressing ESC should only take the one video out of fullscreen that's currently in focus. Instead, both exit fullscreen.
If you instead click on the "<site> is now full screen Exit Full Screen (Esc)" popup (see attachment), only that video exits fullscreen (as it should).
Since that popup explicitly states that you can also use Esc to do the same thing, pressing Esc should not result in different behavior.
No other browser I've tested (Chrome, Edge) behaves like this.
This issue is really annoying imo. I often have a video running in the background, e.g. podcast, music, news, etc, on a secondary monitor. When I then want to watch something on my main monitor, exiting fullscreen on that main monitor also takes the secondary monitor's video out of fullscreen. Esc is an incredibly useful hotkey in many applications with a uniquely consistent effect, the usability of which is being hurt by this issue.
