Thunderbird is a standalone mail and newsgroup client.
A bug was found in the way Thunderbird handled cookies when loading content over HTTP regardless of the user's preference. It is possible that a particular user could be tracked through the use of malicious mail messages which load content over HTTP. The Common Vulnerabilities and Exposures project (cve.mitre.org) has assigned the name CAN-2005-0149 to this issue.
Users of Thunderbird are advised to upgrade to this updated package, which contains Thunderbird version 1.0 and is not vulnerable to this issue.