Sometimes (when your are developing) you may want to force fetching all content fresh all the time including images, resources such as style sheet etc. To facilitate this you can do the following:

1. Open a new window or tab in Firefox.
2. Type about:config in the address bar.
3. Search for “cache” in the search bar and look for network.http.use-cache in the filtered results.
4. Double-click it will toggle it from “true” to “false”. Default should be “true”.

And you’re all set.

Sometime you want to force the cache on one particular page\request. You can do that by holding the “Shift” key while clicking reload or F5.

Cheers

From http://mythinkpond.wordpress.com/2011/06/22/how-to-turn-off-firefox-browser-cache-during-development/