Mozilla has released a new version, 17.0.4., of its Thunderbird E-mail client, for Linux, Windows, and Mac OS X. This is primarily a security-fix release; the new version corrects a critical vulnerability in the HTML editor component. The Release Notes don’t say much more.
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.