One of the best features in the Thunderbird remix called Postbox was the deep and fluid integration of social networking contacts into the desktop e-mail client. Thunderbird itself has begun to get a taste of that power with Thunderbird Contacts, a version of Mozilla Contacts, a Firefox add-on that made the browser more socially aware.
Thunderbird Contacts is so new that Mozilla is still switching between that name and "Contacts for Thunderbird" in its blog post announcing the add-on. Whatever it's called, it pulls your cloud-stored contact info from numerous sites into Thunderbird, and then merges the contacts it identifies as identical. Contacts currently works with Gmail, Yahoo Mail, Twitter, Facebook, LinkedIn, and Plaxo, as well as the Mac address book.
Once the add-on is installed, it can be configured under Tools > Contacts. From there, you can selectively add login information under the Services tab, manage and view addresses under Contacts, and configure contact discovery under the Finders tab. There's also a Permissions tab for easily revoking permission to update contacts without deleting the information you've already downloaded. At the bottom of the Services tab there's options for Emptying your address book, Exporting contacts, and Deleting the data that the Finders tool discovered.
In my tests, the add-on didn't work well with the corporate global address book here, although it did successfully import contacts from Twitter and Gmail into Thunderbird. For Thunderbird fans, this looks like it could grow into a must-have app for adapting the platform-specific "e-mail" into a more generic "communication", but the add-on still feels somewhat rough and has a way to go before that happens.