Obsolete
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
In order to create a skin for mozilla, you will need to know three things. How to edit images, extract zip files, and how to modify CSS. Mozilla uses standard gif, png, and jpeg images for the buttons and CSS to style everything else in the interface.
What is a skin?
A skin does not totally change the interface, it just defines how it looks. You cannot 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 Mozilla, you'll have to look into modifying the chrome. That is beyond the scope of this document.