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.
The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.