Firefox has no built-in option for blocking or filtering sites. A few options for you. # Install the FoxFilter add-on, the downside is add-ons are fairly easy to circumvent - https://addons.mozilla.org/firefox/addon/foxfilter/ # Another add-on that can filter content is the ProCon Latte Content Filter - https://addons.mozilla.org/firefox/addon/procon-latte/ # Install a 3rd-party parental control program such as the free K9 web protection - http://www.k9webprotection.com/ # Use the OpenDNS service, there is a free version, and with it you can filter content based on categories that you choose - http://www.opendns.com/