In order to create a skin for Firefox, there are three things you need to know: how to edit images, how to extract zip files, and how to modify CSS. Firefox uses standard GIF, PNG, and JPEG images for the buttons and CSS to style everything else in the interface.
A skin does not totally change the interface; instead, it just defines how the interface looks. You can't change what happens when the user right clicks on an image, but you can change the look of the right click menu (Make it blue with pink polka dots, for example). If you want to change the functionality of Firefox, you'll have to look into modifying the chrome, which is beyond the scope of this document.