Xulrunner and Gecko relationship

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 开发者_JAVA技巧Mozilla framework.

I'm trying to understand: XULRunner uses Gecko? Or they have nothing to do with each other?

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Gecko is the codename of Mozilla Corperation's rendering engine. It renders web pages. A browser however, needs to do more than just render HTML! It needs to: decode images, network HTTP transfer, etc... Is all done by OTHER components. Necko is one of those other components (for networking).

There is another product, XULRunner. Which embeds Gecko, Necko, and other libraries to form a complete 'product' which many of Mozilla products (including Firefox itself) are based off of.

This is firefox, in Mozilla terminology:

• Gecko + Necko + libpr0n
  • XULRunner
    • Firefox (has it all)

Firefox is merely a frontend of XULRunner. And Gecko is the webpage renderer (a library) which XULRunner utilizes to display webpages.