Just perform a system update in order to get the latest version of Thunderbird
Canonical published details about the Thunderbird vulnerabilities in its Ubuntu 13.10, Ubuntu 12.10, and Ubuntu 12.04 LTS operating systems.According to the company, a number of security issues have been fixed in Thunderbird.
A use-after-free issue was in Thunderbird. If a user had scripting scripting, an attacker could potentially exploit this to cause a denial of service via application crash, or to execute arbitrary code with the privileges of the user invoking Thunderbird.
For a more detailed description of the problems, you can see Canonical's security notification.
The flaws can be fixed if you upgrade your system(s) to the latest Thunderbird package specific to each distribution. To apply the patch, run the Update Manager application.
In general, a standard system update will make all the necessary changes. A restart will be necessary in order to implement them.