The proxy server I have to use at work is screwed, it keeps serving up stale versions from the cache which is especially obnoxious on dynamic sites like Perl Monks. I don’t hold much hope of the proxy server ever being fixed but Firefox lets you work around it. Type about:config in the nav bar look for browser.cache.check_doc_frequency and set the value 1. This forces a compare for the page for every page load.
This (mis)behaviour is discussed on Slashdot and the Firefox preferences are documented at mozilla.org.