1) Click the hamburger menu in the top right of the screen

2) Scroll down the menu and click on "Options"

3) On the left hand side of the screen, click "Privacy & Security"

4) Scroll down to History in the middle of the page and click "Clear History"

5) In the "Time range to clear" dropdown menu, select "Everything". 

A warning will advise that this cannot be undone (this is OK). 

6) Make sure that at least “Cookies” and “Cache” are selected.  Then click “OK”.

7) Close and reopen Firefox.