Using the new logical assignment operators in a JavaScript switch statement could have caused a type confusion, leading to a memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-23954
• https://bugzilla.mozilla.org/show_bug.cgi?id=1684020
• https://www.mozilla.org/security/advisories/mfsa2021-03/
• https://www.mozilla.org/security/advisories/mfsa2021-04/
• https://www.mozilla.org/security/advisories/mfsa2021-05/