Then restart the firewall and check /var/log/messages
# service iptables restart
# tail /var/log/messages
But what are you trying to fix? As far as I know the kernel modules address matters of passive FTP in case your client is behind a NAT interface. FTP is a two way connection. The server requests the IP of the client to connect back to the client for data transfer. In passive FTP mode the client initiates both connections to the server. As far as I know this affects your ability to connect to the FTP server, but not write access to the FTP server as such.
Not necessarily. If the FTP server is behind NAT or Firewall, since the OP asked about relevant modules, then it is often configured to use a passive port. The problem with NAT is that the FTP server may not be able to connect back to the private IP reported by the FTP client, which is typical for external access, hence the connection will fail.