That's great news. Do you know what release this will land in?
Edit: Oops, I guess it's already there. I couldn't think of a site with an appropriately sized fixed header to test it, but Google+ is such a site and pagedown works perfectly there. Nice.
Works for me on FF17. Does not work on Safari or Chrome.
BusinessWeek.com articles don't totally work. They seem to have an extra pop with their header. So 2 levels of BS. Can't totally fault the browser there.
Techcrunch.com works perfectly fine too.
So yes, fixed in FF17 and also seen to work on FF18.
Edit: Oops, I guess it's already there. I couldn't think of a site with an appropriately sized fixed header to test it, but Google+ is such a site and pagedown works perfectly there. Nice.