I'm not totally sure, but this seems to happen when I scroll down past a bunch of comments on a video, and get closer to the text box on the bottom. I've never had it happen on the wiki or the forums or the homepage.
Firefox 18.0.2, Windows 7.
I blacked out my favourites toolbar, but yeah, the text-editing bar will suddenly jump up to the top of the page. This seems to happen sometimes at a certain point when I'm scrolling down through a full page of video comments. If I scroll back up the page, it disappears at that same point (presumably reattaching itself back to the text box below).