Trouble connecting to certain websites...

Hi all, first time poster, long time lurker here.

I recently made the (wise) move to the Mac side with the new iMac. When browsing certain websites, I realize there will be occasional addresses that do not load properly. I get the "Problem loading page" & "unable to connect" error message here on Firefox. The same thing happens with Safari, so I'm pretty sure it's not a browser-specific issue. When I go to the same websites on my PC, they seem to load fine. Any ideas as to what's causing the problem? Is this a Mac OS X issue with certain website designs?

Specifically the error message looks like this:
Unable to connect
Firefox can't establish a connection to the server at *******.com.
* The site could be temporarily unavailable or too busy. Try again in a few moments.
* If you are unable to load any pages, check your computer's network connection.
* If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web.

Thanks for any help guys!
-AG