Mozilla Firefox before 3.0.19, 3.5.x before 3.5.9, and 3.6.x before 3.6.2; Thunderbird before 3.0.4; and SeaMonkey before 2.0.4 do not properly manage reference counts for option elements in a XUL tree optgroup, which might allow remote attackers to execute arbitrary code via unspecified vectors that trigger access to deleted elements, related to a "dangling pointer vulnerability."