Do a web search. Many such programs are free.

I'm sure there are ever so many website-screening programs that are "free," if "free" means not having any upfront dollar cost. But I fear that, like much that is "free" on the internet, it will come with all kinds of behind-the-scenes costs in terms of my privacy. Consider Facebook and Google as prime examples of such services. What I was hoping to find was a screening program that was either offered directly through Mozilla or that was recommended by people in the Mozilla "community," so that I could have a little more trust in it.