Connection to Firebird DB

Hi,
I'm testing Pentaho server CE 5.0 and I'm trying to create a Database Connection to a Firebird db. I'm using Windows 8.1; the Pentaho server and the Firebird server run on the same machine.
I've copied the jaybird-full-2.2.4.jar inside the lib directory of Tomcat.
I've started the CE server and I've defined a connection using "Generic Database".
The settings I'm using are:
custom connection URL:
jdbc:firebirdsql:<server>[/<port>]:/<database-file>

custom driver class name:
org.firebirdsql.jdbc.FBDriver

and the user name and password.
When I test the connection I get this:


Errore di connessione al database:
org.pentaho.di.core.exception.KettleDatabaseException:
Error occured while trying to connect to the database


Error connecting to database:(using class org.firebirdsql.jdbc.FBDriver)
could not initialize class org.firebirdsql.gds.impl.GDSFactory


org.pentaho.di.core.exception.KettleDatabaseExce


I've tried the other JDBC URL as indicated in the Infocenter (JDBC Type 4, as per the jdbc driver), but it doesn't work.
Did anybody try to connect to a Firebird DB successfully?