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→What is Chrome?
In a nutshell, Chrome is the user interface of Mozilla products. The user interface is composed mostly of [[XUL]], [[XBL]], CSS, and JS files. (You can make changes to these files without needing to recompile the Mozilla project you're working on - which is an enormous timesaver, especially for people who are just starting out on Mozilla-based development).
If you look in the chrome subdirectory of your installation, you'll find .jar files - these contain the chrome files that make up the user interface. These jar files are actually just ZIP archives. Extensions are (usually) chrome applets. 90% of an [http://developer.mozilla.org/en/docs/Extensions extension] is , typically is chrome.
In the XUL world, Chrome is the top level window which contains groups of UI elements of various types. Example of chrome are the browser window and dialog window. The chrome obtains its UI description from four types of providers: