Mozilla Firefox before 23.0, Firefox ESR 17.x before 17.0.8, Thunderbird before 17.0.8, Thunderbird ESR 17.x before 17.0.8, and SeaMonkey before 2.20 use an incorrect URI within unspecified comparisons during enforcement of the Same Origin Policy, which allows remote attackers to conduct cross-site scripting (XSS) attacks or install arbitrary add-ons via a crafted web site.