The new FF doesn't work for me. It just sits in the system tray like it's minimised - but you can't actually get it in to a "window". It looks like it's actually just displaying a FF icon and nothing more.
I just downgraded to ver 56 and set it to not update for now.
My laptop did the same thing so I don't know what's up here. I even tried disabling all my add ons - but it still does the same.