First step is to follow the instructable:
Step 1: Changing the Icon.
-If you want it to look like a firefox app, paste this into the "Look for icons in this file" Box.
C:\Program Files\Mozilla Firefox\firefox.exe
-Click on the firefox icon you want it to look like.
-To make the icon the internet explorer icon, enter %ProgramFiles%\Internet Explorer\IEXPLORE.EXE Instead of C:\Program Files\Mozilla Firefox\firefox.exe
*If you want it to look like any other icon, Instead of entering C:\Program Files\Mozilla Firefox\firefox.exe, Enter the programs .exe file and it should bring up the icon you want.
-Last step is to delete or put the original shortcut in a file and re-name the shutdown icon to firefox or whatever app you want.
*Remember the code you need to put into the run dialog to cancel it is: shutdown -a