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/mfsa2022-22/
https://www.mozilla.org/security/advisories/mfsa2022-21/