Certain input to the CSS Sanitizer confused it, resulting in incorrect components being removed. This could have been used as a sanitizer bypass. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.
https://www.mozilla.org/security/advisories/mfsa2020-56/
https://www.mozilla.org/security/advisories/mfsa2020-55/