Mozilla Firefox before 18.0, Firefox ESR 10.x before 10.0.12 and 17.x before 17.0.2, Thunderbird before 17.0.2, Thunderbird ESR 10.x before 10.0.12 and 17.x before 17.0.2, and SeaMonkey before 2.15 do not properly implement quickstubs that use the jsval data type for their return values, which allows remote attackers to execute arbitrary code or cause a denial of service (compartment mismatch and application crash) via crafted JavaScript code that is not properly handled during garbage collection.