I just stumbled upon SyncKolab for Thunderbird. It seems like the perfect solution for getting two-way contact synchronizing working for Zimbra/Thunderbird. However, there is one very important snag - the contacts folders visible over IMAP are read-only.
Is there any way to enable writing to the contacts folder on Zimbra? If not, is this something that might be planned. I haven't been able to find much regarding this in the forums, wiki, or bugzilla.
PS: I don't use it, but supposedly this same extension will allow one to sync calendar entries if Lightning is installed.