A Mozilla extension is an installable enhancement to the Mozilla browser that provides additional functionality (for example Linky, which adds an item to the context menu for opening multiple links in a document or selection). This tutorial walks you through the process of building a Mozilla extension that adds an icon to Mozilla's status bar showing the current status of the Mozilla source code (i.e. whether or not the latest version of the code is compiling successfully and passing tests). The extension will access Tinderbox, mozilla.org's webtool for tracking source code status, to get the status of the code.