Ubuntu Security Notice 582-2 – USN-582-1 fixed several vulnerabilities in Thunderbird. The upstream fixes were incomplete, and after performing certain actions Thunderbird would crash due to memory errors. This update fixes the problem.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/64423/USN-582-2.txt
Source: https://packetstormsecurity.com/files/64423/Ubuntu-Security-Notice-582-2.html