Interesting - this doesn't ever happen to me, and I'm also using Firefox (1.5), navigating exactly as Fred describes.
I
have been having increasing problems with Firefox at other sites, where it gets distracted by random widgets embedded in pages, and I have to kill the tab (and occasionally even the browser itself) to get its attention again. Seems like it's been acting goofy ever since I upgraded to the 1.5 beta (though now I'm using the release version). That probably doesn't have anything to do with what you're seeing, but I felt like venting about it.