[1]: https://en.wikipedia.org/wiki/Usage_share_of_operating_syste...
I'm surprised only 84% of the traffic is Windows users.
You are also implying that developers using windows are all second rate.
When even the basics are wrong you're not getting refreshed much. Especially when adding three letters ('mdn') to the query at the end gives you a much better resource by all metrics.
If you subtract that quotient from the general pool of "Windows developers" it becomes much more of a fair comparison.
Yes, there's useful things on w3schools, but like picking through a trash heap full of rusted bicycles and used syringes, it's a dangerous expedition. You might pick up some awful bad habits along the way.
Actually, the implication is that second rate developers are Windows users. Which is a very different statement.
That's not referring to Windows users in particular.
edit: In response to your edit:
>Making the best web browser is not Mozilla's goal. Their goal is to "promote openness, innovation & opportunity on the Web"
If Mozilla's goal isn't to make the best web browser then no one will end up using their browser, and if no one uses their browser then they wont be able to "promote openness, innovation & opportunity on the Web", no market share = no voice. During the days of IE6's dominance Mozilla got its large user base because it was the best browser available. Mozilla is in danger of hemorrhaging users if it continues making these sort of decisions.
Us "hackers" got Mozilla its user base, we started using it first and convinced our family, friends, workmates/workplaces to switch. If the power users leave, the rest will be soon to follow.
[1]http://www.w3schools.com/browsers/browsers_os.asp