Yesterday, in addition to the release of Firefox 17.0, Mozilla released version 17.0 of its Thunderbird E-mail client, for Linux, Mac OS X, and Windows. The new version incorporates a number of bug fixes and performance improvements. It also fixes 12 identified security vulnerabilities; Mozilla rates the importance of five of these as Critical, and seven as High. Also, as with Firefox 17.0, Mac OS X 10.5 (Leopard) is no longer supported. 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 from the Thunderbird download page.