Change Firefox desktop icon

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I have been trying to change the desktop image for fire fox for a while. Everyone says to right click and choose properties. i have know to do that for years but the issue is there is no properties option when you right click fire fox. i think it has to do with the fact that i have the latest version. it is the same way with itunes. if some one could tell me how to change it for the latest version that would be helpful.