but isn't XUL getting phased out? back in 2008-09, ported a bunch of Delphi apps to XUL. Took longer to develop than Delphi but with mozrepl and a live connection to emacs, it wasn't a bad experience at all. js-ctypes made talking to native libs fairly easy without having to resort to XPCOM extensions. Sure, XUL felt over-engineered and was heavy but it is (was?) an acceptable cross-platform toolkit for a large class of applications. Gecko embedding API was another thing that Mozilla started and then dropped it. Once Chrome arrived on the scene, Moz just turned into a Wile E. Coyote... sigh.
The last time I checked, Etoille used "Pragmatic Smalltalk" built on top of their Objective-c runtime and their Language Kit. It's actually a compiler that generated code compatible with Objective-C. More details can be found from David Chisnall's presentation at FOSDEM a few years ago.