When exiting fullscreen mode, an iframe could have confused the browser about the current state of fullscreen, resulting in potential user confusion or spoofing attacks. This vulnerability affects Thunderbird < 91.10, Firefox < 101, and Firefox ESR < 91.10.
https://www.mozilla.org/security/advisories/mfsa2022-20/
https://bugzilla.mozilla.org/show_bug.cgi?id=1756388