Ports that were written as an integer overflow above the bounds of a 16-bit integer could have bypassed port blocking restrictions when used in the Alt-Svc header. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.
https://www.mozilla.org/security/advisories/mfsa2021-16/
https://www.mozilla.org/security/advisories/mfsa2021-15/