We have a mess of debug APIs now: The debug component, over in extensions/layout-debug, has nsIDebugObject. The layout code, in layout/base/public, has nsILayoutDebugger. In addition, some debug stuff doesn't use either API, e.g. nsIPresShell->SetPaintFrameCount(). I see no reason to maintain the layout-debug extension. Since the code is debug-only, we should just put it in layout.
Of course there is nsIFrameDebug too.