If I scroll to the very top of the page, it goes back where it is supposed to be (I'm very slightly scrolled down in that screenshot). Also, I believe that happened right after I posted in that thread, so I'm wondering whether the text editor going away after replying in some way caused the issue. Using Windows 7. Firefox 21.0.
Kinda similar to this other thread I made, in that depending on where you scroll on the page, sometimes it anchors itself to the right location, other times it doesn't: