Yesterday, in addition to the release of Firefox 18.0, Mozilla released version 17.0.2 of its Thunderbird E-mail client, for Linux, Mac OS X, and Windows. The new version incorporates a number of bug fixes; it also fixes 19 identified security vulnerabilities. Mozilla rates the importance of 12 of these as Critical. (Incidentally, if you were wondering, there was no version 17.0.1 of Thunderbird.) More details are available in the Release Notes.
Because of the security fixes incorporated in this release, I recommend that you update your systems as soon as you conveniently can. You can use the update mechanism built into the software (Help / About Thunderbird / Check for Updates), or you can get a complete installation package, in a variety of languages, from the Thunderbird download page.