Here you can find Firebird documentation. It includes an official Firebird references and guides and articles from IBSurgeon.

Firebird Language Reference (for Firebird 2.5)

in English:  (beta-version is available, it will be updated): Download PDF or  Browse HTML

in Russian:  (release version): Download PDF or Browse HTML

Firebird Hardware Guide

In English:  Download PDF or Browse HTML

In Russian:  Download PDF