Mozilla is a company developing many of the most popular applications you can find, including the popular Firefox browser and mail client Thunderbird. A discussion that started a few days ago on the mozilla.dev.planning list has given birth to the idea of taking all this one step further by building a complete Desktop Environment, like gnome or KDE. Such a thing would then be based on XUL (XML User Interface Language) and it sounds like a very potent venture. This would then include expanding the features of, for example, Firefox to give the browser the ability to do local file management, just like Konqueror.
"I'm only proposing that we make a desktop environment, the same thing that KDE and Gnome, are. The only reason I mentioned a "Mozilla OS" is because we'd probably also want to release distributions for one of more specific kernels as a whole OS, including with it Firefox, Thunderbird, and any other applications built with the Mozilla platform available then. Currently there are plenty of kernels to put a Mozilla desktop environment on."