In specific HSTS configurations an attacker could have bypassed HSTS on a subdomain. This vulnerability affects Firefox < 122, Firefox ESR < 115.7, and Thunderbird < 115.7.
https://www.mozilla.org/security/advisories/mfsa2024-04/
https://www.mozilla.org/security/advisories/mfsa2024-02/
https://www.mozilla.org/security/advisories/mfsa2024-01/
https://lists.debian.org/debian-lts-announce/2024/01/msg00022.html
https://lists.debian.org/debian-lts-announce/2024/01/msg00015.html