You might need git for this script and php 

$ sudo apt-get install git-core php5-cli
$ git clone  git://github.com/mariuz/firebird_scripts.git
$ cd firebird_scripts 

This script builds firebird 2.5 from source 

$ sudo  php firebird-build.php

This script builds nginx and fastcgi support for it 

$ sudo php nginx-build.php