IE/Firefox Problem

I have a fealing that this is a problem with my lan, but whenever I go to some sites (i.e. eBay, apple, mapquest) I get an error on Firefox that says “…It cannot be found” like it doesn’t even exist. I thought it was my firewall, but i disabled it, and the problem still occurs. It’s funny because I only have this problem with a few sites, and they are all huge corporate sites.

Any help on this would be fantastic.