I have been on for four hours and twenty minutes today and have not had "the inability to connect to the net using Firefox after a few hours problem" since I last reported it yesterday. All in all, I have had about 10-12 straight hours of uninterrupted connection to the net so, I feel that I can re-confirm that Web Shield is indeed causing the problem as was my initial impression. The last time I was able to run this long without a problem was the last time I ran avast! without Web Shield.
How really important is Web Shield anyway? I have Zone Alarm, Spybot S&D, Windows Defender, and avast! (without Web Shield) running really smoothly on Vista Home Premium. The system runs beautifully (very fast). I have been considering moving to Avira as a last resort, but I am not sure that they have a component equivalent to Web Shield at this point, and am uncertain that I would be gaining much with the move. We seem to be at the end of the line here, what would you guys do? Again, thanks so much to you all for your help and your kind manner.