...
The customclass attribute allows you to override one of the standard component rendering classes with a custom class for that element. The String value of the customclass element should be the name of the handler that is mapped to the custom classname in the framework's loading engine.
...
Since the XUL textbox element renders an implementation of the XulTextbox interface, then the class that my "variabletextbox" name maps to must also implement the XulTextbox interface. For an understanding of what interfaces are implemented for which elements, download the source and inspect the implementations of the XulLoader interface. The XulLoader impl classes hold the references to the handler mappings (element to implementing renderer Java class name).
This is the most common way to "inject" a customized component into the framework.