Some of us at Mozilla Labs have tried to fill this gap through the Chromeless project (https://github.com/mozilla/chromeless), but we've stopped working on it in favor of a more general runtime for web apps (that's what I was talking about in the last sentence: https://wiki.mozilla.org/Apps/WebRT).
The very fact that XULRunner/Chromeless didn't take off like PhoneGap did in the mobile arena for example is evidence that there's room for innovation in this space.
Happy to elaborate on this, but only if you promise you'll be nicer :)