In addition to releasing a new version of Firefox today, Mozilla released a new version, 14.0, of its Thunderbird E-mail client for Mac OS X, Linux, and Windows. The new release includes a number of bug fixes and performance improvements. It also fixes 11 identified security vulnerabilities; Mozilla rates the severity of five of these as Critical, and three as High. Further details are in the Release Notes.
If you have enabled automatic checking for updates, Thunderbird should inform you of the new version. Otherwise, you can obtain the new version via the built-in update mechanism (Help / About Thunderbird / Check for Updates), or you can download a complete installation package, in a variety of (human) languages.