I have other problems with ZD, related to backing up data stored in the "offline folder". But I've been told that those concerns will be dealt with in a planned update. I think you should file an RFE for your issue with ZD.
As for TB, I'm not sure how necessary it will be to develop a plugin. It would mainly be for synching contacts with Zimbra, right? I think some of the methods at
Sharing address books - MozillaZine Knowledge Base might work for this. Note especially Zindus; I think it's been discussed on the forum here and has been updated to support Zimbra. Furthermore I think GnR is supposed to include CardDAV support, which may work with some more of the options on that page.