Firebird Rocks man, see this link Firebird Advantages over PostgreSQL
Mature Windows support. Firebird is supported on Windows for a very long time and it's well tested. PostgreSQL supports Windows natively since 8.0. Still only a few months...
Mature ADO.NET provider. Npqsql (PostgreSQL ADO.NET provider) is still in beta. Firebird ADO.NET provider supports the embedded Firebird, services API (backup, restore, statistics, batch SQL execution...).
Embedded version. Embedded version (with 2 MB runtime and easy switching to a standalone server) seems to be one of the biggest advantages of Firebird.
Licensing. LGPL (Npgsql is covered by LGPL) seems to be less commercial-friendly than IPL (at least it is a much less clear language).
The Firebird open source database server wins a SourceForge Community Choice Award (in the "Best Project for Enterprise" category).
Bye.