I have Mozilla Firefox for my internet browser. What do I do in effort to block access to a certain website?

Theres an extention for Firefox called BlockSite that'll block access to sites on a list.

https://addons.mozilla.org/firefox/3145/

There is also the ProCon extention which can block pages featuring certain keywords.

https://addons.mozilla.org/firefox/1803/

These tools are very simplistic and shouldn't be used if you're looking for some sort of locked-down parental control software. (And even then, 'nannyware' cannot replace proper supervision)

Hope this was helpful.