CVE 2006-6503

Mozilla Firefox 2.x before 2.0.0.1, 1.5.x before 1.5.0.9, Thunderbird before 1.5.0.9, and SeaMonkey before 1.0.7 allows remote attackers to bypass cross-site scripting (XSS) protection by changing the src attribute of an IMG element to a javascript: URI.

See the CVE page on Mitre.org for more details.