In addition to its release of Firefox 7.0 today, the Mozilla organization has released a new version 7.0 of the Thunderbird E-mail client, for all platforms (Windows, Mac OS X, and Linux). The new version contains numerous fixes and improvements for user interface components, attachment handling, and the Address Book; it also incorporates the newest version of the Gecko layout engine. More details are available in the Release Notes.
You can get the new version via the built-in update mechanism, or you can get versions for all platforms in many languages from the download page.
Update Wednesday, September 28, 15:55 EDT
Mozilla has now posted the list of vulnerabilities fixed in Thunderbird 7.0.