On Tuesday, August 30th, Mozilla pushed out emergency updates for both Thunderbird 3.1 and 6.0. These updates revoke the root certificate for DigiNotar due to fraudulent SSL certificate issuance [Additional Details]. Thunderbird 3.1 users can update via Help > Check For Updates. Thunderbird 6.0 users can update via Help > About Thunderbird. Also updated versions of Thunderbird 3.1 and 6.0 can be downloaded:
Note: Since Thunderbird 3.1 uses the old release process, all features, bug fixes, improvements, etc.planned for the upcoming Thunderbird 3.1.13 release on September 27th are being rolled over to Thunderbird 3.1.14.
Source: Mozilla Developers News