Mozilla Messaging release the scheduled update for Mozilla Thunderbird 3.1.x branch on March 14, 2012. This release addressed security issues.
Thunderbird 3.1.X user can update via Help > Check for Updates or user can download and manually install Thunderbird 3.1.20 form here. Next and final release for Thunderbird 3.1 will be Thunderbird 3.1.21 on April 24th, 2012.