On a PC, click the "Firefox" menu in the top left corner. Next, select the right arrow next to "History >", and click "Clear Recent History"
Make sure "Details" is expanded, then select "Cache" from the list. Uncheck everything else.
In the "Time Range to Clear" drop down, select "Everything".
Select "Clear Now". Your computer will work for a moment, and the process will be complete. You've successfully cleared Firefox's Cache!