Mozilla Thunderbird is a standalone mail and newsgroup client. Several flaws were found in the processing of malformed HTML mail content. An HTML mail message containing malicious content could cause Thunderbird to crash or, potentially, execute arbitrary code as the user running Thunderbird. Several flaws were found in the way malformed HTML mail content was processed. An HTML mail message containing malicious content could execute arbitrary JavaScript in the context of the mail message, possibly presenting misleading data to the user, or stealing sensitive information such as login credentials. A flaw was found in the way Thunderbird handled error responses returned from proxy servers. If an attacker is able to conduct a man-in-the-middle attack against a Thunderbird instance that is using a proxy server, they may be able to steal sensitive information from the site Thunderbird is displaying. Note: JavaScript support is disabled by default in Thunderbird. None of the above issues are exploitable unless JavaScript is enabled. All Thunderbird users should upgrade to this updated package, which resolves these issues. All running instances of Thunderbird must be restarted for the update to take effect.