How to identify firefox private window or not?

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http://superuser.com – I am using firefox 22.0 and some times i select "New Private Window" option from the file menu but i cannot distinguish between private window and the normal window. They both look same.. In chrome, when you select Incognito mode we see a person with hat spying on the window top left corner.Is there any such symbol for firefox rather than remembering?? (HowTos)