If you close Firefox, then start Firefox in Safe Mode, does the problem still occur? For instructions on starting in Safe Mode, see Diagnose Firefox issues using Troubleshoot Mode.

Thanks, Chris - I restarted in Safe Mode, same problem - "Server not found...Firefox can't find the server" However, I have since tried to open on Internet Explorer with same result, but was able to open using Firefox (and IE) on a different PC attached to the same router. So, it apparently is a problem with something else on my PC, not just Firefox.

Try the solutions in Firefox and other browsers can't load websites