It does not mention Firebird, but it's an interesting reading

""The open source database market is immature compared to the market for
proprietary products from the likes of Oracle and IBM," Andrews said.
"We haven't had that test of time where you can really measure which is
more secure. But there's no question that we're seeing an appetite for
open source."

Here is the full article:
http://searchsecurity.techtarget.com/originalContent/0,289142,sid14_gci1137482,00.html

Artur