Through use of reportValidity() and window.open(), a plain-text validation message could have been overlaid on another origin, leading to possible user confusion and spoofing attacks. This vulnerability affects Firefox < 93, Thunderbird < 91.2, and Firefox ESR < 91.2.
https://www.mozilla.org/security/advisories/mfsa2021-47/
https://www.mozilla.org/security/advisories/mfsa2021-45/