Parental control in Windows just lets me set time limits/limits on what programs/games I can run as far as I know (just double checked microsoft.com and that's what they list as well). Given that and my understanding of the Fx feature means that Fx understands that this is a machine with parental controls enabled and _shares that with the world_.
That is nonsense. It's crap. The world cannot (as I stated before) decide what is okay and what not. Going with your porn example (I .. kinda expected that): I'd be entirely fine with a 16yo kid to look at porn. I might not want that very same kid to use the computer around 2am in the night. For the latter I can use parental control features. You (and Mozilla) force your (weird, from this pov) moral/set of values on me. Enabling parental control does NOT mean that I want the world to know about it. It also doesn't imply that porn isn't okay. On the other hand, maybe I would put the NRA website on the list of things I don't want to see in my household. No one can decide what is okay or not - except for the parent/admin.
"There is no UI in Firefox, because Firefox uses the OS parental control features to enable/disable it" is missing the point. Unless I fail to understand what this does I cannot use the parental controls without Firefox/Mozilla abusing that flag and asking the YouPorn administrators for parental support.
This is a misfeature and at best useless, although I'd lean towards harmful and wrong.