Download this icon file:
http://i.tylian.net/spitfire-icon256.ico

Right click Firefox and select Properties.
http://i.tylian.net/DGXPxI.png

On the Shortcut tab select Change Icon.
http://i.tylian.net/2X5dgD.png

Select the icon file you downloaded above and click Ok.
http://i.tylian.net/BQAGUS.png