Mozilla Developer Iain Ireland discovered a missing type check during unboxed objects removal, resulting in a crash. We presume that with enough effort that it could be exploited to run arbitrary code. This vulnerability affects Thunderbird < 68.9.0, Firefox < 77, and Firefox ESR < 68.9.
https://www.mozilla.org/security/advisories/mfsa2020-22/
https://www.mozilla.org/security/advisories/mfsa2020-21/
https://www.mozilla.org/security/advisories/mfsa2020-20/