There was a way to change the value of JavaScript Object properties that were supposed to be non-writeable. This vulnerability affects Firefox < 144, Firefox ESR < 115.29, Firefox ESR < 140.4, Thunderbird < 144, and Thunderbird < 140.4.
https://www.mozilla.org/security/advisories/mfsa2025-85/
https://www.mozilla.org/security/advisories/mfsa2025-84/
https://www.mozilla.org/security/advisories/mfsa2025-83/
https://www.mozilla.org/security/advisories/mfsa2025-82/