If you're finding that your Firefox browser is not caching any images (css/javascripts) and loading them multiple times on the same page, check the following cache settings of Firefox by typing in about:config in the browser's address bar.

Both browser.cache.disk.enable and browser.cache.memory.enable values should be set to true in order to enable browser caching in Firefox. You can double-click on the name to toggle the value between true and false.