Type about:config in the address bar > press Enter - click 'Accept the risk and continue' if that appears - search for browser.cache.disk.capacity > lower the value - search for browser.cache.disk.smart_size.enabled > change the value to false - restart FF To turn off disk caching altogether, set browser.cache.disk.enable to false. I've done this with no noticeable effect. Try it if you have a high-speed connection and don't use a lot of RAM for other apps.