Mozilla Firefox 4.x through 10.0, Firefox ESR 10.x before 10.0.3, Thunderbird 5.0 through 10.0, Thunderbird ESR 10.x before 10.0.3, and SeaMonkey before 2.8 do not properly restrict write access to the window.fullScreen object, which allows remote attackers to spoof the user interface via a crafted web page.