I decided to upgrade my Ubuntu server from 13.04 to 13.10. When finished, I lost the ability to remotely connect to my Firebird 2.5 database (works locally / 127.0.0.1)!
I didn't change anything in the configuration and I don't understand why this has happened.
I use Webmin remotely to configure the firewall (IpTables) and the other resources in the server. I have several other services that work correctly!