Postgres vs Firebird?

As a long-time user of Postgres, (First started using it at 7.0) I'm reading
recently that Firebird has been taking off as a database.

Perhaps this is not the best place to ask this, but is there any compelling
advantage to using Firebird over Postgres? We have a large database (almost
100 tables of highly normalized data) heavily loaded with foreign keys and
other constraints, and our application makes heavy use of transactions.

I say this as my company's growth has been exponential, showing no sign of
letting up soon, and I'm reviewing clustering and replication technologies so
that we can continue to scale as nicely as we have to date with our single
server. (now with a load avg around .30 typically)

-Ben
--
"The best way to predict the future is to invent it."
- XEROX PARC slogan, circa 1978