> Does anyone use firebird with Zope? If so:

Yes we do.
Currently its a ZOPE 2.6.1 and Firebird 1.0.3 together on Windows machines.
As DB connection layer we make use of kinterbasedbDA (http://www.zope.org/Members/mwoj/kinterbasdbDA)

This combination performs well (even using large BLOBS), is stable and used in prodictions environs.

Best
Bjoern