Firefox
1.Click the Menu (three stacked horizontal lines), click History, then click Clear Recent History….
NOTE:
You can also access this menu by holding Ctrl + Shift + Delete.
2.Click on the drop down box to the right of Time range to clear: and select Everything. Then, click on Details and check both Cookies and Cache. Click on Clear Now.
Warning
Please be noted that clearing cache might log you off all your current accounts.
The original source of this guide can be found here.