Start Firefox by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Refresh

Are you sure the Question Mark(?) isn't there in the last line of the Three Bar Menu? Hovering over it shows Help. Why do you wish to Refresh? Many contributors consider this to be a last resort.

Hit the Alt key to reveal the Menu Bar. The Help menu can be accessed there.