Mozilla has released Thunderbird versions 12.0 which fixes a number of critical security issues including potential memory corruption. All Thunderbird users should apply the update as soon as possible.
For more information, see
Note that Thunderbird 12 introduces auto-updating features. For areas where auto-updating may break application compatibility, Thunderbird ESR may be preferred. See http://www.mozilla.org/en-US/thunderbird/organizations/ for more information. For Thunderbird ESR admins, the equivalent release is Thunderbird 10.0.4esr.