Mozilla Firefox before 24.0, Firefox ESR 17.x before 17.0.9, Thunderbird before 24.0, Thunderbird ESR 17.x before 17.0.9, and SeaMonkey before 2.21 do not properly identify the "this" object during use of user-defined getter methods on DOM proxies, which might allow remote attackers to bypass intended access restrictions via vectors involving an expando object.