A locally-installed hostile program could send `WM_COPYDATA` messages that Firefox would process incorrectly, leading to an out-of-bounds read. *This bug only affects Firefox on Windows. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 78.11, Firefox < 89, and Firefox ESR < 78.11.
https://www.mozilla.org/security/advisories/mfsa2021-26/
https://www.mozilla.org/security/advisories/mfsa2021-24/