Mozilla Messaging has release a stability and security update for Thunderbird 3.1.x on January 31st, 2012 with Thunderbird 3.1.18 release. Existing Thunderbird 3.1.x users can update within Thunderbird via Help > Check for Updates or can manually download and install Thunderbird 3.1.18 from here. With the launch of the new Thunderbird ESR, Mozilla has announced: Thunderbird 3.1.x will be maintained with security and stability releases until April 24th, 2012 with the Thunderbird 3.1.20 release.
