You can create a "whitelist" which only allows access to specified sites but if you allow google for instance it will allow access to all sites they go to through the google link. So not particularily good and is done through the connection tab in options.
There is a downloadable extension http://procon.mozdev.com/
but don't know how effective it is.
You could also try something like "NetDog" a filtering tool aimed at parents and downloadable from my favourite site http://www.majorgeeks.com it's free as well. Or if you don't mind paying something like Cyberpatrol.
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