Integer overflow in Mozilla Firefox 3.5.x before 3.5.11 and 3.6.x before 3.6.7, Thunderbird 3.0.x before 3.0.6 and 3.1.x before 3.1.1, and SeaMonkey before 2.0.6 allows remote attackers to execute arbitrary code via a large selection attribute in a XUL tree element, which triggers a use-after-free.