Use-after-free vulnerability in the nsAnimationManager::BuildAnimations function in the Animation Manager in Mozilla Firefox before 24.0, Firefox ESR 17.x before 17.0.9, Thunderbird before 24.0, Thunderbird ESR 17.x before 17.0.9, and SeaMonkey before 2.21 allows remote attackers to execute arbitrary code or cause a denial of service (heap memory corruption) via vectors involving stylesheet cloning.