A web page could trick a user into setting that site as the default handler for a custom URL protocol. This vulnerability affects Firefox < 136, Firefox ESR < 128.8, Thunderbird < 136, and Thunderbird < 128.8.
https://www.mozilla.org/security/advisories/mfsa2025-18/
https://www.mozilla.org/security/advisories/mfsa2025-17/
https://www.mozilla.org/security/advisories/mfsa2025-16/