A use-after-free vulnerability can occur when the SMIL animation controller incorrectly registers with the refresh driver twice when only a single registration is expected. When a registration is later freed with the removal of the animation controller element, the refresh driver incorrectly leaves a dangling pointer to the driver's observer array. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.
https://www.mozilla.org/security/advisories/mfsa2019-11/
https://www.mozilla.org/security/advisories/mfsa2019-08/
https://www.mozilla.org/security/advisories/mfsa2019-07/
https://bugzilla.mozilla.org/show_bug.cgi?id=1531277