You can Install and use the following addon, it should have the function you require:

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

Or else have a look at this link to view other addons with similar functions and choose which one you prefer.

https://addons.mozilla.org/en-US/firefox/search/?q=parental+control&cat=all

Good luck

I'd go with that from the way the question was asked, and blocksite is a small download 16.4 KB download and easy to setup and use, has a password (one could turn off extensions, but the list should be protected), the really neat thing is that it blocks the links as well they just show up as plain text no link markings. You need to specify the exact prefix http://sss.yyy.com you can't just specify yyy.com It will protect against accidental use in Google and you can turn off the cache link off through Google itself. You could also include http://webcache.googleusercontent.com in your blocklist filter to turn that link to black also (but best to suppress so it doesn't show).

Everyone should also have "Adblock Plus" extension installed with filter subscriptions -- Adblock Plus by itself does nothing -- it needs filters, and you can create your own (no protections against change). A hosts file on your system is also important but a bit unwieldy to use as you have to pick up each variation -- example.com, www.example.com, joes.example.com but it applies to all applications. (hosts file need admin authority).