Check your Windows Firewall preferences and see if it isn't blocking Firefox from showing websites. Also have you checked your internet connection? If your internet connection is working fine yet you can not access any website then it can also have an error. If so I recommend you uninstall and install it again. Sometimes in the process of downloading it can come corrupted, then if possible, download again the newest version of Firefox and install it. But do not forget to check the Firewall.