A recent Hacker News submission asked what an "acceptable" cross-platform GUI toolkit would be. It discusses common complaints about the existing toolkits GTK+, Qt, Tk, and wxWidgets. It suggests creating a new toolkit with the qualities of being written in C, keeping it simple, LGPL licensing, easily skinnable for different OS's, binding for scripting languages, and be "simple and easy to use".Good news! I know of an existing GUI toolkit that fits this bill. Its written and extensible in C/C++, has liberal licensing, is very skinnable, has bindings for JavaScript, Python, Java, Perl, and Ruby, and lots of existing open source code that you can learn from. It even has a large, mixed platform install base, so you know that the bugs are minimal. Its based on an object brokering system called XPCOM, and an application framework called Mozilla, but often goes by the name Firefox. Given the popularity of web applications in the startup community, Firefox seems like an obvious option for desktop apps. Yet it hasn't caught on, and I'm not sure why.I recently worked on a desktop application for my day job. Given their limited interface goals and a game-oriented 3d engine, they had decided to write the interface directly in OpenGL. It was difficult to design interfaces like that, so I did a prototype integration with wxPython. That worked well enough to start real product development, during which I realized that wxPython had many of the "cross-platform toolkit" woes like needless internal complexity, difficulty in writing new components, and general ugliness.I spent some time contemplating how people wrote successful cross platform desktop applications, and the two things that struck me were (a) there aren't many, (b) except Firefox, (c) and this might underly the popularity of web applications. So I did a new prototype using Firefox with a XUL user interface. What I found was that Firefox worked out great as a standalone application with modular code design, but it was difficult to write good interfaces in XUL. XUL seems to be well tested only as far as the functionality in Firefox is concerned. We switched to Ext-JS, started pretending to be web app devs, and got down to implementing features.Writing a desktop app based on XULRunner has some interesting side-effects. For one, it makes retargeting your code as a web application or Firefox plugin really easy. It also lets you develop using a variety of hacker friendly languages. Like programming in lisp, it changes your perspective on how applications should be developed, and makes questions like "which UI toolkit is the best?" seem fundamentally flawed. In an age when Apple is rewriting their apps to run in a browser (and still look and feel like desktop apps), isn't writing a code against a desktop UI toolkit fighting the tide?