Mozilla Thunderbird is a standalone mail and newsgroup client. It was found that a Certificate Authority issued a fraudulent HTTPS certificate. This update renders any HTTPS certificates signed by that CA as untrusted, except for a select few. The now untrusted certificates that were issued before July 1, 2011 can be manually re-enabled and used again at your own risk in Thunderbird; however, affected certificates issued after this date cannot be re-enabled or used. All Thunderbird users should upgrade to this updated package, which resolves this issue. All running instances of Thunderbird must be restarted for the update to take effect.