You could get the icon from a release of Firefox if you have software that can extract icons from applications. Alternately, find a page with a Firefox logo that is big enough and resize it and save it to an .ico file.

Sounds like a question for a Windows 7 support forum. I don't use Win7, but in WinXP if you right-click a desktop icon and open Properties, there is a button for Change Icon. As finitarry alluded to, you will need to come up with your own icon file of the correct size. Keep in mind that Nightly updates frequently, actually it updates nightly, and you may find that you need to replace that icon every day. Speculation on my part, I have never wasted my time replacing a desktop shortcut icon for a program that updated frequently. And if you spent any time searching for how to do that specifically for Firefox, anything you might have found about fiddling Firefox program files or the Firefox Profile folder hasn't worked since the Firefox 3.0 version.

Do as instructed above, and select the Nightly program as the source of the icon. It may be that it has other icons to choose from. I know that Firefox does. This is how I changed my icon.