Mozilla Firefox before 3.5.12 and 3.6.x before 3.6.9, Thunderbird before 3.0.7 and 3.1.x before 3.1.3, and SeaMonkey before 2.0.7 do not properly restrict the role of property changes in triggering XUL tree removal, which allows remote attackers to cause a denial of service (deleted memory access and application crash) or possibly execute arbitrary code by setting unspecified properties.