Everybody has his or her own reason to hack Firefox. Mine is usually: “just for fun”.
So, “just for fun”, and to show how far you can go with the extension mechanism, here is a little useless extension I wrote: a Nintendo Wiimote driver for Firefox.
What does it do? It brings Wiimote events to web content. You can change tabs with a “forehand/backhand” tennis drive and, in your web page, make your elements move using Wiimote events (rotation, g-force, position, etc.). Web pages, of course, do not support this API. But, with another extension, such as aJetpack or Greasemonkey, you can “hack” a website to add support for the Wiimote.
although the extension is open-source, it works only for Linux currently. Check out this video: