Close all open Firefox windows and tabs. Open a new Firefox window.
Select "History" (in some versions of Firefox, you ma need to select the "Firefox" menu in the upper left-hand corner, then hover your mouse over "History" until the drop-down menu appears)
Select "Clear Recent History."
Select the items to delete (be sure to include the item for "Cache") and verify that the time range to be cleared is "Everything."
Now click on the "Clear Now" button.