Firebird is a free open source database with a license that lets you download and use it free of charge.
It means that Firebird can be used in any commercial software absolutely free, without any restrictions or conditions.
However, the development of Firebird requires funding, which is organized through the Firebird Foundation - the non-profit organization that raises the funds for Firebird development. 
To support Firebird development, please donate a reasonable amount to the Firebird Foundation.