Just as they've recently updated Firefox to Firefox 8, the Mozilla team released today an update to their email client, Thunderbird. The major new feature in Thunderbird 8 is its blocking of automatic add-on installations from third parties.
By handling the third party installs itself, Thunderbird, like Firefox 8, increases security. Thunderbird 8 also sports a new Add-ons Manager and will suggest incompatible add-ons for you to disable if you wish, to improve stability.
Several other improvements are listed in the "What's New in Thunderbird" tab, including increased responsiveness and start up speed.
Check for the Thunderbird update (Help > About Thunderbird) if you haven't been automatically updated already. The official Thunderbird page is as of now still listing version 7.01, however, so if you want to download the latest version of the email client right away, look to Softpedia.