I'm not interested in Firebug, a XUL debugger, or a JavaScript editor, but a true WYSIWYG IDE for XUL form design.
I think no Wysiwyg IDE available for Xul but there is Spket IDE, it may help you
I use XUL Explorer for doing quick mock-ups of things or checking out elements with which I'm not familiar. It's not WYSIWYG, but you can drag and drop elements.
Spket IDE has no WYSIWIG either.
You may want to try Extension Developer at https://addons.mozilla.org/en-US/firefox/addon/7434, which is a Firefox add-on. One screenshot on that page illustrates an WYSIWIG XUL editor. You can edit an XUL file with Spket IDE and load it with this editor to check the final effect.
You might want to try Songbird's XUL layout editor. I recall that it is very complete and easy to use, and it comes with lots of examples of every single XUL element put in multiple layout scenarios. I believe you need a nightly build or a dev release of it, which you can easily get from the website.
The good thing is that besides the editor, it comes with some other fancy and very useful XUL developer tools. And of course, it is a great iTunes replacement ;)
I just wanted to note that I do XUL development with ActiveState KomodoEdit. It doesn't have WYSIWYG XUL gui design but it recognizes XUL files and can give you code hints for the XUL namespace as well as their corresponding JS files.
Note that Komodo was written in XUL as well. It works very well for me :-)
Here is a MozillaWiki page with a list XUL Integrated Development Environments and descriptions of each.