Cross-site scripting (XSS) vulnerability in 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 allows user-assisted remote attackers to inject arbitrary web script or HTML via a selection that is added to a document in which the designMode property is enabled.