Hello, Sheila!
My name is Slava and I'm from Russia. Unfortunately, I got the same problem with FF.
ERROR
The requested URL could not be retrieved

While trying to retrieve the URL: http://amarillo.com/

The following error was encountered:

Access Denied.

Generated Sat, 23 Jun 2012 08:10:20 GMT by sms3.cac004.groupz.net (squid)
You've done a great work checking all the ways you've mentioned. I did all the same and there's no result. I think it has something to do with the latest updates of FF too. I'll try to instal a newer version that is Firefox 14.0 (Beta 8) and then, if it won't give a positive result, I'll try one of the previous versions of FF.