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.

The 13th Firebird Developers Day will take place in Piracicaba-SP, Brazil, in 9-July. Registrations are open at http://www.FirebirdDevelopersDay.com.br. There are less than 50 seats available right now, so hurry up and register asap! You can check the videos recorded by the speakers, where they talk about their FDD sessions here.

Em português:
O 13º Firebird Developers Day acontecerá em Piracicaba-SP, no dia 9 de Julho. As inscrições estão abertas, e restam menos de 50 vagas, portanto, se increva agora em http://www.FirebirdDevelopersDay.com.br para não ficar de fora! Vários palestrantes gravaram chamadas em vídeo falando sobre suas palestras no evento, que podem ser conferidos.

FB/Java is an External Engine plugin for Firebird that makes Firebird capable of run functions, procedures, and triggers made in the Java platform.

Download it now:https://github.com/FirebirdSQL/fbjava/releases

It complements Jaybird making it interface with Firebird engine and handling the infrastructure necessary to support user routines. It also has a client utility with functions to install and uninstall the plugin in a database and those users may use to deploy and un-deploy Java classes and resources stored in JAR files to a database.

Bugs should be filed in the GitHub project. This test version uses a modified snapshot of Jaybird 3.0.