Mozilla has released a new version, 13.0.1, of its Thunderbird E-mail client, for all platforms: Mac OS X, Windows, and Linux. This release fixes several bugs, but does not appear to include any security patches. More details are available 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.