If you look in your Mozilla install directory, you will find the chrome directory, beneath which are the XUL descriptions for the user interface. Mozilla interprets the chrome URL to find the actual file it wants, adjusting for such things as the current locale. It turns out that a little bit of configuration to your web server is required in order to get Mozilla to recognise XUL files. Some of the widgets aren't properly implemented yet in Mozilla, and it can vary from platform to platform. Layout Mechanism Arrangement of widgets in a XUL interface is done primarily by use of boxes .
Boxes can have their contents stacked either horizontally or vertically.
Layouts can be constructed by a combination of these boxes. Each UI element has a flex attribute, which governs how it will stretch to fill the space available. There is also a spring element, which is an empty box you can use to put space in between other UI elements. As the window containing these elements was expanded, each button and the spring would all enlarge to consume the new space in equal measure, owing to their having identical flex values. Note the use of CSS styles to specify minimum widget dimensions.