The license for the Firebird itself and the ADO.NET provider is very flexible: Initial Developer's PUBLIC LICENSE Version 1.0 and INTERBASE PUBLIC LICENSE Version 1.0. Both are modified versions of Mozilla Public License v.1.1. That means you can use it freely in your commercial applications:

You can include the binaries in your commercial product but you have to tell your users where they can get the sources (of Firebird/provider). Any changes you make to Firebird's source code must be made available in source code under the same license. Your proprietary code can remain proprietary and closed.

Consult your counsel for more details or read the license in detail.