Not only unable to find a way to block sites on Firefox, I tried using the content advisor in my pc's control panel. Set a password, entered the sites I want blocked, but Firefox apparently doesn't interface with the content advisor.

So, I resolved this myself. Content Advisor in the control panel of the pc does work, but only for internet explorer. For Firefox, there is an addon designed for parents called WebFilter Pro that I installed. Once installed, I had to go to particular sites the kid had been visiting and block it. Then I realized that I could "change security policy" (that's an actual link) and I blocked all the dating sites, social sites, chat rooms, etc. Once done, I was surprised to see that the add-on disappeared so the kid can't so easily alter the settings. The one flaw is that it does not block facebook's secure url (https://). I will point that out to them as a smart kid could get in anyway.