Contacts for Thunderbird is the Mozilla Labs extension that was first developed for Firefox and now ported to Thunderbird.
Thunderbird Contacts lets you pull in your “web contacts” from sites like LinkedIn, Facebook, and Twitter. It also lets you connect to your mail contacts inside of Yahoo or GMail. As each contact is extracted from a site, the Contacts add-on tries to merge the contacts from different sites together into a single contact on your system.
I have played with Contacts for over a year and have been waiting for its integration into Thunderbird. Watch it in action: pure awesomeness! Go get Contacts for Thunderbird.