Firefox does not currently have site blocking 'built-in." However, there are some add-ons that can help:

ProCon Latte - https://addons.mozilla.org/en-US/firefox/addon/1803/

"ProCon is a content filter for the Firefox browser. It can filter any kind of material (pornography, gambling, hacking, cracking, etc...), it can also block all traffic, making sure that only desired websites (set in the Whitelist) can be accessed, and includes a profanity filter, all *like* a parental control filter.

ProCon also has password protection in order to keep *others* from changing the settings."

FoxFilter - https://addons.mozilla.org/en-US/firefox/addon/4351/

"FoxFilter helps block content for essentially any user-defined Web site or criteria. You can quickly and easily block content for an entire site or enter custom keyword filters (e.g. videos, movies, images, pornography, nudity, gambling, explosives, etc.) that will be used to help block content for any site that contains these keywords. There are many pornography-related keywords already included! (see Filter Keywords below for more info). FoxFilter is designed to work with Firefox on Mac, Linux and Windows and includes password-protected settings and security features to help prevent it from being bypassed, uninstalled or disabled. A password recovery feature is also available."

Add-on search for Parental Controls -https://addons.mozilla.org/en-US/firefox/search/?q=parental+control&cat=all&lver=any&pid=1&sort=&pp=20&lup=&advanced=

Also see Parental Controls

Of course, those will only work for users using Firefox. Users browsing the web with other browsers will not be affected.

The HOSTS file is the most efficient way to block websites for all users on the Operating System for all browsers used on that PC, "Portable" browsers included. And is is easy to prevent other users from accessing that file, too. http://en.wikipedia.org/wiki/Hosts_file http://www.ssmedia.com/utilities/hosts/