Mozilla Thunderbird is a standalone mail and newsgroup client.
The RHSA-2011:1243 Thunderbird update rendered HTTPS certificates signed by a certain Certificate Authority (CA) as untrusted, but made an exception for a select few. This update removes that exception, rendering every HTTPS certificate signed by that CA as untrusted. (BZ#735483)
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.