Mozilla Firefox before 16.0.2, Firefox ESR 10.x before 10.0.10, Thunderbird before 16.0.2, Thunderbird ESR 10.x before 10.0.10, and SeaMonkey before 2.13.2 do not prevent use of the valueOf method to shadow the location object (aka window.location), which makes it easier for remote attackers to conduct cross-site scripting (XSS) attacks via vectors involving a plugin.