I can't comment on Firefox issues...but it would seem that you now have a system connection problem.
Have you tried going through any XP troubleshooters (HELP menus) to check anything or get some clues?
Have you tried searching Event Viewer for clues?
How To Use Event Viewer - http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
Have you tried uninstalling your networking device, rebooting and then reinstalling same?
Have you checked Device Manager to see if there are any known hardware problems?