Please help! Firefox not loading websites properly?? Frustrating

Firefox has recently taken to not loading certain websites properly. By this I mean there is no real formatting on the website and the only things visible are hyperlinks, so it's like the site map. There are no radio buttons, banners, colors, or layout. The only websites that seem to avoid this problem are Mozilla.com, Wikipedia, and Google. Yahoo, Twitter, and most other sites are all messed up. I've already cleared my cache many times, reinstalled Firefox, tried the new beta version of Firefox and nothing is working. This is extremely frustrating because I can't find any support and I can't navigate through websites anymore! == Operating system == Windows XP (SP 2)