Select the "Firefox" menu in the upper left-hand corner, then hover your mouse over "Options" until the drop-down menu appears and select "Options" from the drop-down menu.
Click on the "Privacy" tab if it is not already selected and the click on the "clear your recent history " link:
*Be sure to uncheck all items except for "Cache"* and set the time range to be cleared to "Everything"
Now click on the "Clear Now" button.