CVE 2010-5074

The layout engine in Mozilla Firefox before 4.0, Thunderbird before 3.3, and SeaMonkey before 2.1 executes different code for visited and unvisited links during the processing of Cascading Style Sheets (CSS) token sequences, which makes it easier for remote attackers to obtain sensitive information about visited web pages via a timing attack.

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