The Mozilla Messaging project has released a new version, 3.1.5, of the Thunderbird E-mail client. This version fixes a number of bugs affecting performance and the user interface; it also patches eight security vulnerabilities, several of which are rated Critical. There is more information in the Release Notes.
Because of the security patches, I recommend installing the new version as soon as you conveniently can. You can get the new version via the built-in update mechanism (Menu: Help / Check for Updates), or you can download installation packages for all platforms — Mac OS X, Windows, and Linux — in many different languages.