Firefox sometimes ran the onload handler for SVG elements that the DOM sanitizer decided to remove, resulting in JavaScript being executed after pasting attacker-controlled data into a contenteditable element. 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