Mozilla: X-Frame-Options header ignored when duplicated (MFSA 2012-51)

Mozilla Firefox 4.x through 13.0, Firefox ESR 10.x before 10.0.6, Thunderbird 5.0 through 13.0, Thunderbird ESR 10.x before 10.0.6, and SeaMonkey before 2.11 do not properly handle duplicate values in X-Frame-Options headers, which makes it easier for remote attackers to conduct clickjacking...