Ow there are two, actually!

Firefox Inside

Put chrome://browser/content/browser.xul in your address bar. And tada! It’s Firefox in Firefox! (It does use more memory though! Find out more about chrome://)

The Book of Mozilla

Again, put about:mozilla in your address bar. You will be presented with the Book of Mozilla, 7:15:

There’s an interesting survey about Firefox and browsers in general that you can take part in. It is part of a Master thesis. It has nice questions and covers most a Firefox enthusiasts would ask. Go on and take the survey!

PS: The website has nice Javascript animation and layout!