First either confirm or manually enter your network settings anew.
How to troubleshoot your connection:
Press "windowskey+R" tpye in "cmd" and click the Ok button to start the command line.
Enter "ping www.onrpg.com" and press enter.
You can get a result that is similar to one f these 2 examples:
(If you get an error instead of these 2 outputs, please post the error message here.)
If there is no working connection (second option, 100% loss) to onrpg.com, try a second command
"ping 212.72.60.180"
If you get a connection there, but non with "www.onrpg.com" you have a DNS-lookup problem. That means that your DNS server address is not correctly set in your connection settings.
If you do not get a connection with both, your internet doesn't work.
If you have a fine connection with both, it's an issue with Firefox.
Use this link to troubleshoot:
http://kb.mozillazine.org/Error_loading_websites
EDIT:
I remember, when upgrading to windows 7, the wizard explicitly told me to remove AVG beforehand.
I did remove it and reinstall it after the upgrade.
Did you do that too?
In case you ignored the warning, your installation of windows might be broken.
I am not up to date on repair mechanics of W7 (not had the chance to test, xD), but you should probably remove AVG, repair windows, try to get the newest version of AVG online with your hopefully working Internet and install it again.