An unspecified parser-utility class in 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 does not properly handle EMBED elements within description elements in RSS feeds, which allows remote attackers to conduct cross-site scripting (XSS) attacks via a feed.