Apps similar to XUL Explorer include XUL Gear, which is free and open source. Other options are XulBooster. Between them they cover Mac, Windows and Linux.
XUL Gear is a powerful and easy to use extension that plays the role of an XUL editor. XUL Gear allows Firefox developers to write less xml code and to arrange XUL objects in a visual form ( in the WYSIWYG manner).
XulBooster is an Eclipse Feature adding XUL support to the Eclipse IDE and WTP. The Feature includes Editors, Views, Wizards, Snippets, Help, Builder etc. you can develop with it firefox extension, xulrunner application for example.
