Thunderbird only: Thunderbird extension Google Contacts automatically syncs contacts between your Google and Thunderbird address books. The sync is bi-directional, and the extension is smart enough to detect Gmail accounts you’ve already set up in Thunderbird, so setup is a breeze. Beyond that, there’s not much to it—Google Contacts does exactly what it sets out to do. Now that you can easily sync any desktop calendar with Google Calendar and sync Google Contacts with extensions like this one, it’s easier than ever to keep your Google data synchronized. Thanks Pranesh!