$ sudo apt-get update
$ sudo apt-get install firebird2.5-classic 

firebird2.5-classic is installed in your system.

Make ensure the firebird2.5-classic package were installed using the commands given below,

$ sudo dpkg-query -l | grep firebird2.5-classic *

You will get with firebird2.5-classic package name, version, architecture and description in a table

firebird2.5-classic package basic information: