Gecko:Event Dispatching Coordinates
From MozillaWiki
Currently, when event coordinates are dispatched through the Gecko frame system, the x and y coordinates in the event represent the offset of the event from the nearest view for the current frame. This is stupid because it is very complicated to translate the event correctly when you pass it from one frame to another. The event coordinates should simply be relative to the frame on which you call HandleEvent or whatever.
sharparrow1@yahoo.com is working on some of this.