The gContactSync add-on synchronizes your contacts between Gmail and Thunderbird's address book.

Thunderbird extension Zindus syncs your Google contacts with Thunderbird's address book...

Harness the to-do-managing power of Remember the Milk from inside your mail reader with an alpha extension for Thunderbird.

ReviewGist looks like a great place to check out the broader merits of a product before making a big purchase

Windows/Linux (Thunderbird): Import Outlook's PST files and choose which email folders to bring into Thunderbird with PST Import, a free extension for the open-source, cross-platform email manager Thunderbird...

Spicebird improves employee productivity with its cleanly integrated e-mail, calendaring and instant messaging through intelligent design

An extension to add addressbook-related options to the context menu.

"Here's how to get the full Gmail experience in Thunderbird with IMAP..."