I did a little bit of research and narrowed down to two possible files that I would need to make changes to add the new properties(movementX/Y) to the MouseEvent object:
I declared the properties on the IDL file, and defined the getters on the cpp file.
After building….. it worked!!
It seems a bit unreal to think that we are implementing a new feature on firefox, looking 3 months back, the thought of modifying firefox never crossed my mind.