The inkscape:bbox is defined as the global coordinate system. However, the center.x and center.y must be the coordiante system of the parent group of the SVG entity. Therefore, we need to do coordinate transformation from the global coordination system to the local coordination system.

content	MadBuilder	content/
overlay	chrome://browser/content/browser.xul	chrome://MadBuilder/content/MadBuilderOverlay.xul