Firebird is an open source SQL relational database management system that runs on Linux, Microsoft Windows, Mac OS X and a variety of Unix. The database forked from Borland's open source edition of InterBase in 2000, but since Firebird 1.5 the code has been largely rewritten
Firebird comes in two flavors, called architectures: Classic Server and Superserver. Which one should you install? That depends on your situation. A short overview of the most important differences follows.