Affix bad positioning at bottom of page (in Firefox)

Created by: hero-m

How to reproduce: Use getbootstrap.com or latest master. Open the "Components", or "Javascript" page. Scroll to bottom of page, using keyboard keys (down or page-down). Affix covers a part of footer, and still has "affix" class, not "affix-bottom".

Somehow, this wont happen anymore if you move a little up and then try coming down again, unless you move back to the top of the page and repeat.