Close all open Firefox windows and tabs. Open a new Firefox window. Now select the "Firefox" menu in the upper left-hand corner, then hover your mouse over "History" until the drop-down menu appears and select "Clear Recent History" from the drop-down menu.
Uncheck all items except for "Cache" and verify that the time range to be cleared is "Everything."
Caution
Be sure to uncheck all items other than "Cache"
Now click on the "Clear Now" button.