There are various sites trapping you in fullscreen to trick and/or force you into responding to permission prompts. This is most commonly done with add-on prompts, which is why we want to disallow add-on prompts from being shown in fullscreen at all (bug 1412561).

Another more general problem is that these prompts can be easily spoofed in fullscreen, because we are lacking a browser chrome.

The add-on use case is clearly malicious, while there may be a valid reason to request other permissions for fullscreen apps, which is why I think it's too much to outright forbid it.

All in all, I think it's sensible to force-exit fullscreen and disallow re-entering while a permission prompt is shown.

Might need to be mindful of bugs like bug 1355000 when implementing this.

Pushed by archaeopteryx@coole-files.de:
https://hg.mozilla.org/integration/autoland/rev/0b99dfb9f7c7
Remove permission prompts when entering full-screen and leave full-screen when a permission prompt is shown. r=johannh

https://hg.mozilla.org/mozilla-central/rev/0b99dfb9f7c7

Posted site compatibility note: https://www.fxsitecompat.dev/en-CA/docs/2019/permission-requests-can-no-longer-be-made-while-in-fullscreen-mode/