The Firebird licenses

Firebird is a free, open-source database management system, but “free” does not mean that everything is permitted. The use of Firebird is governed by two licenses: the IPL (InterBase Public License) and the IDPL (Initial Developer's Public License). The first one covers the parts of the source code that were inherited from InterBase; the second applies to the additions and improvements made by the Firebird Project. Both licenses offer similar rights and restrictions. In short:

Use of the software is free, even for commercial purposes. You may also redistribute the software, separately or with a product of your own, but you may not claim ownership or credit for it. Any license notices included with Firebird must remain intact.
You may modify and recompile the Firebird source code or parts of it. You may distribute such modified versions, but if you do so, you must document your modifications and make them publicly available, at no cost, under the same license as the original code.
You may include Firebird source code (modified or not) in a larger work and distribute that larger work, in source and/or compiled form, under a license of your own choosing. You need not publicize the source code for the entire larger work, but you must fulfill the license conditions for the parts that were taken from Firebird, whether they were modified or not.

Please notice that the above is a simplified overview. Only the original license texts are legally binding. You can find them here:

http://www.firebirdsql.org/ipl/ (IPL)

http://www.firebirdsql.org/idpl/ (IDPL)