Firefox caches web content once it downloads the html pages , images etc. to avoid downloading them, thus improving the browsing speed. At times during development when we make changes on the Server, we still don't see the latest changes immediately as Firefox may be caching. In Other  we may want to test website performance and want to clear out any web cache. In any case, cleaning the Firefox cache is just couple of clicks. Go to Firefox Preferences and then to Advanced Tab, Then Network sub tab and Click on Clear Now. It's easy as that.