Mozilla has released Thunderbird versions 11.0 and 3.1.20, each of which fixes a number of critical security issues including potential memory corruption. Two versions of the update are available depending upon the version of Thunderbird installed. All Thunderbird users should apply the update as soon as possible.
Also, note that the Thunderbird 3.1.x series will no longer be supported after April 24, 2012, so this may be the final Thunderbird 3.1.x release.
For more information, see
(For Thunderbird ESR admins, the equivalent release is Thunderbird 10.0.3.)