1. Open up Firefox
2. Go to 'Tools' on the menu bar
3. Select 'Options'. A window will appear; click 'Privacy' on this window
4. Once the new window opens, click on 'Clear Private Data'
5. Check the box that says 'Cache'
6. Click on 'Clear Private Data Now'
7. After a few moments, close the window. Your cache will now be clear.