Can you connect to the internet with other programs than firefox? From your post I suspect the answer is no, but I just want to make sure I understand what the problem is.
Can you ping an outside network? Run the following code in a terminal and post back the results
ping -c4 www.google.com
if that doesn't work, run
ping -c4 188.8.131.52
also, please post the output from
These last commands need to be run as root.