Newer Versions of Firefox

1. Close all open Firefox windows and tabs. Open a new Firefox window.
2. Click the Menu button in the top right
3. Select the "Options" button (Marked by a gear icon) - On Mac the button will say "Preferences" like below
   
   
   
   
4. On the left select the Privacy tab
5. Click "clear your recent history"
   
   
   
   
6. A dialogue box will appear, select the items to delete - make sure that "Cache" is selected - and verify that the time range to clear is set to "Everything"
   
   
   
   
7. Now click the "Clear Now" button

Older Versions of Firefox

1. Close all open Firefox windows and tabs.  Open a new Firefox window.
2. 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)
3. Select "Clear Recent History."
   
   
   
   
4. Select the items to delete (be sure to include the item for "Cache") and verify that the time range to be cleared is "Everything."
   
   
   
   
5. Now click on the "Clear Now" button.