Hey guys, we're going to write our UI using a markup language, code the behaviours using JavaScript and style it through CSS. Oh, and very new thing, we'll use the browser to display it to make it ubiquitous and even cross-platform.

Does it ring a bell? Yeah, XUL and XPFE... Unfortunately, that's not about XUL...

That's about XAML and W/PFE. Yeah, W/PFE!!! As in "Windows Presentation Foundation Everywhere"!!!

Source: Laurent Jouanneau and Tristan Nitot

Update: to be completely fair with MSFT, it must be said that using a markup language for UI is not a Mozilla thing. MSIE did that long before Netscape/Mozilla, using HTML for all MSIE dialogs.