v1.3 is out for FF3.5 (or 3.0.12) against ZCS 5.0.x
GnR ZCS 6.0.x fix is in the works.
Bug 39549 - ZimbraDnD not compatible with FF 3.5 & GnR http://gallery.zimbra.com/files/uploads/ZimbraDnD.xpi
Edit:
Might have to remove 1.1.1 manually since it creates a 2nd 1.3 entry.
We're not pushing auto-update at this time.