Mozilla developers reported memory safety bugs present in Firefox 80 and Firefox ESR 78.2. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox < 81, Thunderbird < 78.3, and Firefox ESR < 78.3.
https://www.mozilla.org/security/advisories/mfsa2020-44/
https://www.mozilla.org/security/advisories/mfsa2020-43/
https://www.mozilla.org/security/advisories/mfsa2020-42/
https://www.debian.org/security/2020/dsa-4770
https://security.gentoo.org/glsa/202010-02
https://lists.debian.org/debian-lts-announce/2020/10/msg00020.html
https://bugzilla.mozilla.org/buglist.cgi?bug_id=1648493%2C1660800
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00077.html
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00074.html