Zindus synchronizes your Thunderbird addressbook with a Zimbra server.
This is free (open-source) software and runs on all Thunderbird platforms including Windows, Mac OSX and Linux.
The current release is beta, and your feedback is welcome. Any bugs you run into will probably just result in a failure to sync. ie. you are unlikely to lose any data, but it is possible, so backup your addressbook - you’ve been warned!