The good news is it's not a problem on your end, you don't have to fiddle with any settings on your computers. The bad news is that the people responsible for hosting that site probably messed something up. I have just tried to reach that site on a Linux PC from Firefox and it just won't load. So, as it happens once in a while, Windows actually is not to blame. I tried accessing the site both by URL and by IP address, and nothing appears. I monitored the network traffic between my computer and their server and it simply does not respond in any way.

I can't say for sure why the proxy server you used is able to reach the site, but it works for me, too, and pretty quickly as well. In your place I would use the proxy server to go there and if they have some kind of support contact there, you can tell them that you are unable to reach the site from any of your computers without using such a proxy, and a guy from halfway around the world using a different OS has the same problem.

Edit: CoffeeCup wrote his reply while I was writing mine. The observation "doesn't like the IP address" is a possible explanation, but likely because of a misconfigured server on their part. It's not the firewall on your side, not your browser or a plug-in and no virus or other malware, I can exclude all these things on my side.