Let's start simple...First, when you ping the website like yahoo.com, write down the ip address (209.191.93.52). Now type that in the url and if it comes up this time, it's a DNS issue as stated earlier.
There are a number of things that you can do from a command line. I might suggest typing ipconfig /?. This will give you the options available. More than likely, you'll have to perform a flushdns or release and renew of your ip address.
This is Mozilla Firefox's support for the issue