During process shutdown, a document could have caused a use-after-free of a languages service object, leading to memory corruption and a potentially exploitable crash. 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/