First of all tel me about , r u using proxy server?
If yes is it a Linux or in Windows?
If u r using a proxy server....., Just check ur ip-address using
#ifconfig
if it shows ip.., then it means u r connected to LAN, if not then u have 2 configure it.
just give ip-address of ur PROXY SERVER IN MOZILLA BROWSER SETTINGS..
After that ur internet will work
Enjoy
Guarav