When a file download is specified via the `Content-Disposition` header, that directive would be ignored if the file was included via a `<embed>` or `<object>` tag, potentially making a website vulnerable to a cross-site scripting attack. This vulnerability affects Firefox < 140, Firefox ESR < 128.12, Thunderbird < 140, and Thunderbird < 128.12.
https://www.mozilla.org/security/advisories/mfsa2025-55/
https://www.mozilla.org/security/advisories/mfsa2025-54/
https://www.mozilla.org/security/advisories/mfsa2025-53/