The Norton Internet Security software package for Windows includes a firewall component, which controls the programs that can connect to the outside world and the types of connections that they can make. If Norton Internet Security doesn't permit Firefox to connect to the Internet, Firefox generates a "Server not found" error when you try to browse to websites.
To see if you're running Norton Internet Security's firewall:
The newest versions of Norton Internet Security should automatically detect and allow Firefox access to the Internet. Older versions, however, may silently block Firefox after it is updated.
To ensure Firefox can access the Internet, you must remove the Firefox entry from Norton Internet Security and allow it to re-detect Firefox. Below are instructions for Norton Internet Security 2009, 2007, 2008, 2002 and Norton Internet Security 2010. Other versions may have variations on these processes.
(based on a live chat session, some instructions are predicted)
You should now be able to access the Internet with Firefox.
For more information on Norton Internet Security, see Symantec's support site.
Based on information from Firewalls (mozillaZine KB)