A new version of XUL Maker is available. It's a visual IDE for XUL. I think it shows great potential, however I don't like the UI. It's way too complicated. For instance the content area by default is only 25% of the window. The rest is occupied by row upon row of toolbars, a sidebar and a help area.
So I wrote a lengthy piece on what I think should be done to simplify the interface.