Mozilla Thunderbird is a standalone mail and newsgroup client. Several flaws were found in the processing of malformed HTML content. An HTML mail message containing malicious content could cause Thunderbird to crash or, potentially, execute arbitrary code with the privileges of the user running Thunderbird. A flaw was found in the way Thunderbird processed the "Enter" keypress event. A malicious HTML mail message could present a download dialog while the key is pressed, activating the default "Open" action. A remote attacker could exploit this vulnerability by causing the mail client to open malicious web content. A flaw was found in the way Thunderbird handled Location headers in redirect responses. Two copies of this header with different values could be a symptom of a CRLF injection attack against a vulnerable server. Thunderbird now treats two copies of the Location, Content-Length, or Content-Disposition header as an error condition. A flaw was found in the way Thunderbird handled frame objects with certain names. An attacker could use this flaw to cause a plug-in to grant its content access to another site or the local file system, violating the same-origin policy. An integer underflow flaw was found in the way Thunderbird handled large JavaScript regular expressions. An HTML mail message containing malicious JavaScript could cause Thunderbird to access already freed memory, causing Thunderbird to crash or, potentially, execute arbitrary code with the privileges of the user running Thunderbird. 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.