Proxy server

I have been using Mozilla Firefox for several years now with no problems. I recently signed up with AT&T SBC DSL service, and Mozilla worked fine until a few days ago. Now I’m denied access to Mozilla, and the reason given is the proxy server. When I check the settings through Avast, it says proxy server is working OK.
Any suggestions?

What is your operating system ?

Can you expand on the reason given being the proxy server, whos proxy ?
Does your ISP require a proxy to connect, if so what settings ?

I am using Windows 2000 Professional / NT.
When I try to open the Mozilla browser, I get a failure / error message that says “Mozilla is configured to use a proxy server that is refusing connection”.
I cannot figure out how to remove the requirement for Mozilla to use a proxy server, and I do not know what proxy server it is referring to.

Have you manually set a proxy in firefox (see image) ?

With your OS there should be no need to set-up the web shield proxy it is transparent and to all intents an purposes the server shouldn’t now if a (localhost) proxy is being used.