After read:
Do not confuse the Gecko/XULRunner SDK with XULRunner itself. The Gecko SDK is a collection of header files and tools used to develop general XPCOM components which add functionality to the existing platform, whereas XULRunner is a framework which can be used to run standalone or embedded applications based on the Mozilla framework.
I'm trying to understand: XULRunner uses Gecko? Or they have nothing to do with each other?