XUL (XML User Interface Language, or “zool”) is a markup language used to create user interfaces based on XML Mozilla platform, which allows construction of multi-application feature, itself can run independently on application or connect to the Internet.
Similarly to HTML, XUL allows creating interfaces using markup language (markup language), use CSS to the form (appearance) and use JavaScript to describe behavior (behavior). You can also access to the program to read and write through a remote network, or call the Web Services (web services).
XUL offers a collection of widgets used to create the menu, toolbar, the tab panel, or the structure of the tree-level (hierarchical trees) … etc.
The theory is to understand the simple XUL is used to draw the GUI for the application running on Mozzila.
Interface of XUL Explorer like this:
Code by default it does have is a window. Your task is to code the <- your code here ->
Here is an example of extremely simple:

After you have completed the form save the file. Xul (eg test.xul)
When you run this file open with Firefox in mind.