You can use the HOSTS file to block web sites for all browsers on that PC. http://en.wikipedia.org/wiki/HOSTS The best way to do it, IMO. That file is buried in the Windows Operating System, and it's not something a user with limited experience is going to look for if they can't load a website they want to visit. Or, you can use the Block Site extension in Firefox. https://addons.mozilla.org/firefox/3145/ It is very easy to override a Firefox extension by disabling the extension or using the Firefox SafeMode.