I am not entirely certain from your description what you have done but will at least try to give some information that could help.

Did the new PC previously work and connect to the internet, eg by using Internet Explorer ?

You could try making the network tab connections settings similar to those that work in Internet Explorer, or try the default settings see :

• Options window - Advanced panel

You may also be interested in the thread "Firefox says "proxy server is refusing connections" after Mozilla update"

PS and in the article Websites don't load - troubleshoot and fix error messages

Firefox 3.6.4-up versions have the default setting to use system proxy settings. You should change it to No Proxy in Tools > Options > Advanced - Connection = Settings button.