When navigating from inside an iframe while requesting fullscreen access, an attacker-controlled tab could have made the browser unable to leave fullscreen mode. This vulnerability affects Firefox ESR < 91.5, Firefox < 96, and Thunderbird < 91.5.
https://bugzilla.mozilla.org/show_bug.cgi?id=1739220
https://www.mozilla.org/security/advisories/mfsa2022-01/