When I go to, for example, reddit.com ... the page doesn't automatically show the new content or update my message indicator. I have to manually refresh every time. This is using firefox 3.6.16 on windows 7. I have set the browser.cache.check_doc_frequency setting to 1, and it still doesn't fix the issue.
I realized later what's going on. If I close the browser with tabs open, and then reopen the browser so that it reloads those tabs automatically... the tabs don't all get auto-refreshed. So I was viewing e.g. unrefreshed reddit. If I open a new tab, or close all tabs and then visit the page... it loads the freshest version.
I think this was a fluke, as recent visits to the page show it freshly every time. It may have something to do with firefox quitting in a nonstandard way... I use mouse shortcuts that will quit it using either ctrl+W or alt+F4. Maybe one of those doesn't quit properly or cause it to check for an updated version next time.
I don't know why you're having your reddit problem, but to actually answer your question:
You can force Firefox to update any page (and ignore the cache settings) by using CTRL + F5 to refresh the page, instead of just F5.