Ever since I upgraded to Firefox 4 I've been seeing this weird bug. I will follow a link to a site, read the site and then hit the back button, but the back button does nothing. So I look at my history and see that somehow the site I am on still has two to four entries in the history --meaning that I would have to hit the back button some three to five times get away from the page. And that is assuming that more history items don't get added each time I hit "back".
I'm not sure that this is a browser bug. It may be some trick used by the web sites to defraud their advertisers over how many hits they are getting, but I almost never saw it before I upgraded.