In the Mozilla blog post Prefer: safe, I read:
Mozilla believes users have the right to shape the Internet and their own experiences on it. However, there are instances when people seek to shape not only their own experiences, but also those of young users and family members whose needs related to trust and safety may differ.
We’re pleased to announce a smart feature in Firefox for just this type of user called Prefer:Safe, designed to simplify and strengthen the online trust and safety model.
I thought this was encouraging. The “Prefer: safe” was clearly a browser header, and this description sounded like it would indicate to the site the browser wanted increased privacy and security controls, or at least better defaults. I thought it unlikely that a lot of sites would honor it, but maybe it would improve things.