here is the release announcement
Around the world, July 16, 2004 Today, the Firebird Project announced the release of Firebird 1.5.1 for Windows and Linux.
Firebird V1.5.1 is an updated version of Firebird V1.5. The V1.5 release of Firebird represented a major upgrade to the Firebird
database engine, and was released earlier this year.
Firebird V1.5.1 represents a committment by the project to develop and deliver ongoing improvements to this popular open source
database engine.
Downloads are available at:

The NPTL (Native Posix Thread Library) Linux builds referred to in the release notes will be available shortly.