Are you trying to set up your network, or are you trying to configure firefox. There is a book on the www.tldp.org
website on Network Administration. It will give you a good background in how networks work.
You haven't provided any information on your network or even which distro you are using. If you opt to use DHCP, then most items will be configured automatically. In most cases, a Cable or DSL modem or a router will contain a DHCP server.