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 then click on the "clear your recent history " link:
Uncheck all items except for "Cache" and set the time range to be cleared to "Everything"
Caution
Be sure to uncheck all items other than "Cache"
Now click on the "Clear Now" button.