Once your browser is opened, click on the "Firefox" menu in the top left corner and choose "History".
Click on "Clear Recent History".
(Or, you can press Ctrl-Shift-Delete to open the recent history window and skip the previous 2 steps)
In the Time Range to Clear drop down list, choose "Everything".
Make sure "Details" is expanded then select "Cache" and “Cookies” from the list. Uncheck everything else. Then click on "Clear now".
Your computer will work for a moment, and the process will be complete.
You've successfully cleared Firefox's Cache!