How to write an extension for Firefox

Suppose you are techie minded, and are a big fan of Firefox, and want to add to the amount of functionality available for Firefox, one good way to contribute is to write a specific plugin / addon for Firefox that will add some cool new functionality.
But how does one do such a thing ? You will need to know a lot more detail about the process of doing so, and it helps to have a good tutorial available that will help. One location where such a tutorial is located is at this link.
You need to have a basic understanding of JavaScript, and an understanding of XUL, and that should be enough. The tutorial shows you how to do 'Hello World'.