My problem is I need to get the position of the viewport relative to the extent of the entire document. I am only concerned with Firefox. My issue is that everything I have read says that: viewport height is window.innerHeight scroll position is window.pageYOffset document total height is document.height So, I would expect that if I scrolled to the bottom of a page that window.innerHeight + window.pageYOffset = document.height But it doesn't! Can someone please explain to me why this is?