There isn't a built-in feature to prevent you from visiting particular websites, but there are some add-ons designed to do this. I haven't tested these myself but you can check them out, read the reviews, see what you think:

• LeechBlock https://addons.mozilla.org/firefox/addon/leechblock/ - currently a Featured extension
• BlockSite https://addons.mozilla.org/firefox/addon/blocksite/ - has recent negative reviews

Thanks for the help. I used Blocksite (because of the name). It needs some work on the interface. But, it works instantly.