First thing to try is another browser like Internet Explorer because your problem could be your internet connection to your ISP is broken.
If IE works and Firefox doesn't then your problem is Firefox. If neither work then it's your connection and perhaps a cable has been pulled out or your router needs rebooting, if they don't fix it call your ISP.
If it's Firefox faulty then try running it in Safe Mode (a shortcut should be in your Start
Menu). If that works then you probably need a new profile which usually fixes problems better than a reinstall of Firefox. Some help here.....