It's not Firefox. The list of latest posts (http://www.whathifi.com/forum/latest-posts) and the forum landing page (http://www.whathifi.com/forum) are refreshed every two minutes. I'd rather the latest posts page was done every minute (which is pretty much what the old forum did), but the code required to do so is a little bit server-intensive and I haven't refactored it yet to make it safe to run so often.
Any longer than that and it's definitely browser cache, which should be resolved by a Ctrl-R or Ctrl-F5 (eg). Note that the site behaves differently if you're logged in or not - pages aren't cached if you're logged in (other than the two minute rule above), whereas they are if you're not (with me so far?). Cache times are different across the site, with forum lists being cached for less time than, say, the home page, which doesn't change that often.
Does that clarify?