Set-Cookie response headers were being incorrectly honored in multipart HTTP responses. If an attacker could control the Content-Type response header, as well as control part of the response body, they could inject Set-Cookie response headers that would have been honored by the browser. This vulnerability affects Firefox < 123, Firefox ESR < 115.8, and Thunderbird < 115.8.
https://www.mozilla.org/security/advisories/mfsa2024-07/
https://www.mozilla.org/security/advisories/mfsa2024-06/
https://www.mozilla.org/security/advisories/mfsa2024-05/
https://lists.debian.org/debian-lts-announce/2024/03/msg00001.html
https://lists.debian.org/debian-lts-announce/2024/03/msg00000.html