XUL - as we all know, the "XML User interface Language" - is an XML file format.
All UI includes some text, and so do XUL files.
Sample XUL: openLocation.xul
<?xml version="1.0"?>
<dialog ... title="Mozilla - Open Web Location">
...
<description>Enter the web location (URL), or specify the local file you would like to open:</description>
...
</dialog>