Mozilla cites two major advantages of using WebIDE as compared with developing apps for competing platforms. In-browser development tools are already familiar to the enormous number of Web developers that exist, so using them for application development minimizes the number of new tools and new skills that must be learned.
Second, they're extremely lightweight as development tools go. The substantial size of downloading tools such as Xcode or Visual Studio, in addition to the cost of developer licenses on other platforms, can limit their appeal and usability, especially in emerging markets. Putting the tools into the browser means that Mozilla's reach is near universal.