Mozilla Firefox before 16.0, Firefox ESR 10.x before 10.0.8, Thunderbird before 16.0, Thunderbird ESR 10.x before 10.0.8, and SeaMonkey before 2.13 do not properly manage a certain insPos variable, which allows remote attackers to execute arbitrary code or cause a denial of service (heap memory corruption and assertion failure) via unspecified vectors.