Firefox in Firefox!
Here's something of a "Do you know?" variety.
Now, for the more curious among you - the reason why this works is because the chrome:// uri is interpreted by the firefox engine as a reference to its chrome directory and the subsequent paths provided are used to navigate the inner tree structure. The xul file is interpreted as an interface and rendered as such, complete with all event handlers - giving you a complete browser within a browser. To see the innards of Firefox, open up the browser.jar in the chrome directory of your Firefox installation using any standard unzipping utility and look at content/browser/browser.js and content/browser/browser.xul to have a feel of how flexible my favourite browser actually is!