and download firestarter unstable (find it here) and then install it on your linux box...

...although it seems that debian servers are down because of a recent security breach.

I guess you can try to compile firestarter from source code... or read the iptables manual here

You can try this script... I have not tried myself... it is harmless... other than reversibly turning off your firewall... yeah, thats safe nowadays!.

#!/bin/bash
# Flushing and initializing the chains
iptables -F
iptables -X
iptables -Z

# This is intended for the loopback device... you can try it with eth0 (if that #is your network interface, or eth1 or whatever). If this let you get into the #internet, then you have a firewall/iptables issue, easily solved, but you #have to read the manual... if you reboot the iptables parameters will be #restored as they were before you ran this script
iptables -A INPUT -i eth0 -j ACCEPT
iptables -A OUTPUT -o eth0 -j ACCEPT