Yeah I have noticed B4 that firefox seems to have a very persistent cache, if it sees divs that are named it kind of thinks the divs are always gonna be the same. I started putting a 'refresh after a few minutes' metatag on most of my sites see page source on
http://pornlinks.kwikfire.com that solved much of the problem.
I used to get this on ie when I surfed through a proxy and it was the proxy cache that caused the problem, didnt matter how often I refeshed my browser cache it fetched from the proxy cache! Setting a metatag was the only sure cure. I guess if you always went to your page from a bookmark then putting a balnk query string (just a question mark) on the end of the url would force a refresh as well.