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], 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 <b>browser window</b> and <b>dialog window</b>. The chrome obtains its UI description from four types of providers: