By design Firefox will use your disk as cache. However, if you have a good amount of RAM available you could quickly improve Firefox’s performance by telling it disable disk caching and only use RAM.
Here’s how you do it;
in your URL bar type in “about:config”. this will bring up the config page.
Using the filter box, lookup “browser.cache.disk.enable”. Change its value to “False” by double clicking on it.
Using the filter box again, lookup “browser.cache.memory.enable” . Change its value to “True” by double clicking on it.
browser.cache.disk.enable to False<br /> browser.cache.memory.enable to True