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 preferences button (Marked by a gear icon)
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5. On the left select the Privacy tab.
6. Click "clear your recent history"
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8. 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"
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10. Now click the "Clear Now" button

Older Versions of Firefox

1. Close all open Firefox windows and tabs.  Open a new Firefox window.

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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."
   

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2. Select the items to delete (be sure to include the item for "Cache") and verify that the time range to be cleared is "Everything."
   
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3. Now click on the "Clear Now" button.