Due to a layout change, iframe contents could have been rendered outside of its border. This could have led to user confusion or spoofing attacks. This vulnerability affects Thunderbird < 91.8, Firefox < 99, and Firefox ESR < 91.8.
https://www.mozilla.org/security/advisories/mfsa2022-13/
https://bugzilla.mozilla.org/show_bug.cgi?id=1735265