I have to say, I don't really care much for this, and I'm surprised to see it coming from Mozilla.

"Prefer:Safe" is rather vague-sounding. I was expecting something about incorporating something like "HTTPS Everywhere" into Firefox.

I understand the appeal of something to prevent kids from stumbling across porn or violence online, but let's not muddy words like "trust" (which mean something very different when talking about security and privacy) with the concept of age-appropriate content.