I'm confused as to why you've mentioned proxy settings but, just in case it's relevant, let's start there:
In Firefox, go to Tools > Options > Advanced > Network > Settings and ensure that 'No proxy' is selected (unless of course, unusually, you normally connect via a proxy). Click 'OK' twice.
Then check that your desktop is actually connected to your router. (i.e. either that the ethernet cable is plugged in or that there's a wireless connection).
Next, reset the TCP/IP stack:
http://www.windows-secrets.co.uk/2013/08/repair-the-windows-87vista-tcpip-stack/