Relative URLs starting with three slashes were incorrectly parsed, and a path-traversal "/../" part in the path could be used to override the specified host. This could contribute to security problems in web sites. This vulnerability affects Firefox < 120, Firefox ESR < 115.5.0, and Thunderbird < 115.5.
https://www.mozilla.org/security/advisories/mfsa2023-52/
https://www.mozilla.org/security/advisories/mfsa2023-50/
https://www.mozilla.org/security/advisories/mfsa2023-49/
https://www.debian.org/security/2023/dsa-5561
https://lists.debian.org/debian-lts-announce/2023/11/msg00030.html
https://lists.debian.org/debian-lts-announce/2023/11/msg00017.html