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This raises a question: what are 'XPCOM components' components of exactly? The answer is Gecko, Mozilla's standards compliant, embeddable web browser and toolkit for creating web browsers and other applications. XPCOM is the means of accessing Gecko library functionality and embedding or extending Gecko.  Much has been [http://developer.mozilla.org/en/docs/XPCOM written about XPCOM] elsewhere, and you are encouraged to read more on the theory behind this technology in conjunction with this walkthrough.