PROBLEM: You want a clear cache in Firefox.
- Tools --> Options --> Advanced --> Network
- Click the CLEAR NOW button to the right of "Your web cache is currently using x MB of disk space."
- Click OKAY.
NOTE: You can also set your cache level on this option. Look at "Override automatic cache management" and change the number in "Limit cahce to xx MB of space" where xx is the number of MB. I don't do more than 8, but it is your preference.