PS: Replying here since Helen considers this offtopic in fb-sup.

A few reasons that comes to my mind:

FB is much simpler and smaller to deploy.
FB requires almost no "fine-tune" and no-dba to run.
FB doesn't required exclusive access to do "sweep".
Strong and ready-to-help community.
FB has embedded server (not sure if it will be useful for your needs).
Many many options of access methods (JDBC, .NET, Delphi, PHP, etc.)
IMO, FB new features development is more "business oriented"

Anyway, as everything in life, PostgreSQL has its merits too, for sure.

Carlos
http://www.firebirdnews.org
FireBase - http://www.FireBase.com.br

te> For a coming "erp light" project we're deciding between firebird and
te> postgres
te> and just can't make up our minds, have good experience with fb, but
te> firebird is really unknown here in sweden (mysql you know...).

te> Delphi is also considered "dead" around here, so we're not in the
te> BDE-game. Postgres is the main option, anyone around here with
te> good points of view is really appreciated.

te> No flames please,