Contacts is an experiment with a new look on how the Thunderbird address book could evolve. Born from the Mozilla Labs Contacts in Firefox add-on, Thunderbird Contacts was adapted to work in Thunderbird and includes some new features.
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.