Incorrect handling of __proto__ mutations may lead to type confusion in IonMonkey JIT code and can be leveraged for arbitrary memory read and write. This vulnerability affects Firefox < 66.0.1, Firefox ESR < 60.6.1, and Thunderbird < 60.6.1.
https://www.mozilla.org/security/advisories/mfsa2019-12/
https://www.mozilla.org/security/advisories/mfsa2019-10/
https://www.mozilla.org/security/advisories/mfsa2019-09/
https://bugzilla.mozilla.org/show_bug.cgi?id=1538006