When following the value's prototype chain, it was possible to retain a reference to a locale, delete it, and subsequently reference it. This resulted in a use-after-free and a potentially exploitable crash. This vulnerability affects Firefox < 70, Thunderbird < 68.2, and Firefox ESR < 68.2.
https://bugzilla.mozilla.org/show_bug.cgi?id=1577107
https://security.gentoo.org/glsa/202003-10
https://usn.ubuntu.com/4335-1/
https://www.mozilla.org/security/advisories/mfsa2019-33/