1. Click the Menu (three stacked horizontal lines) then click Options.
    Firefox Menu
  2. On the left side, select Privacy & Security, then select Clear Data....
    Firefox Privacy & Security
  3. In the pop-up menu, make sure both boxes are checked for Cookies and Site Data and Cached Web Content, then click Clear.

NOTE: You can also clear cache and cookies using the legacy Clear Recent History menu by holding Ctrl Shift + Del.