From lifehacker.com: "Sure, you could clear your entire browser cache (in Firefox: Tools -> Clear Recent History), restart your browser, or go nuts and restart your computer, but most of the time that's overkill. Any of those methods could be worth trying eventually, but before you try anything more drastic or time-consuming just hold your Shift key and click the Refresh button in your browser (or press Ctrl+Shift+R [Win]/Ctrl+F5 [Win]/Cmd+Shift+R [Mac]; the keyboard shortcut doesn't work on Safari). While a normal page refresh (whether you click the refresh button or use the F5/Ctrl+R/Cmd+R shortcuts) still uses those cached files rather than re-downloading them, the Shift+Refresh clears the cache for the files in that page so that you're sure to be running the latest and greatest version of what that web site's serving.
If That Still Doesn't Work?
A cache-overriding reload isn't a fix-all, but it is the first thing I tell anyone to try ("Hold Shift and click the Refresh button") when they're having a problem with a web site. If it doesn't fix the problems you're having with a web site, you've still got plenty of troubleshooting paths to exhaust (clear your entire cache, restart the browser, restart your computer, uninstall extensions, etc.), but it's a good place to get started.