Do you mean the Firefox shortcut to the application is a blank icon, or do you mean that a shortcut to a web page is a blank? You can change the icons used for different document types.

The short cuts to various urls are blank. If I go to properties of shortcut & go to change icon there are no options other than generic

Is this what you see? Press the Browse button, make your way to the computers Programs Folder, then the Firefox folder. Then select Firefox.exe.