How to enable Firebird to work through network protocol at CentOS 6.x

To enable Firebird to listen to the external requests at CentOS 6.x (ip_address:/path/database.fdb), add to the file /etc/sysconfig/iptables the following line:
-A INPUT -m state --state NEW -m tcp -p tcp --dport 3050 -j ACCEPT
Then run
/etc/init.d/iptables restart
Check the result with commands
netstat -tulpn | less
iptables -L -n

If you are using events, set RemoteAuxPort parameter to some explicit value and add it to iptables too.