How to load XUL into Gecko engine

I was given a visual C++ application based on Gecko and Mozilla 19.0.2 library to render webpages. It works fine for my company for many years. Recently, my company gave me the task to port Mozilla build-in debugger for debugging JS and CSS to our visual C++ application. I have investigated this and found Mozilla 19.0.2 build-in debugger is implemented by XUL, js and jsm, where XUL builds the menu interface, js and jsm builds the underlying onCommand call for the debugger. Since my application uses Gecko engine, my question is how do I load the XUL to the Gelco engine? Any sample code? Thank you for any tips/directions!