Along with the Firefox 10.0 release today, Mozilla has released version 10.0 of its Thunderbird E-mail client, for Mac OS X, Linux, and Windows. The new version includes improved search capability (including the ability to do a Web search), as well as a number of bug fixes. More details are available in the Release Notes.
You can get the new version via the built-in update mechanism (Help / Check for Updates), or you can download versions for all platforms, in more than 50 languages.
Update Wednesday, 1 February, 10:58 EST
The new version also fixes seven security vulnerabilities, five of which Mozilla rates as Critical; the details are here.