The most likely reason is that the system internet settings are not correctly configured. To verify this is the problem run Chrome, Safari or Internet Explorer which all use system internet settings and try opening www.powermapper.com.
Note that Firefox maintains its own settings so that it may work when other browsers don't. If you can't open pages in a browser other than Firefox then there is a system configuration problem. To resolve the problem consult the Microsoft or Apple support pages from your usual browser.