--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -14172,17 +14172,19 @@ nsGlobalWindow::NotifyDefaultButtonLoade
 
   // Get the button rect in screen coordinates.
   nsIFrame *frame = aDefaultButton.GetPrimaryFrame();
   if (!frame) {
     aError.Throw(NS_ERROR_FAILURE);
     return;
   }
   LayoutDeviceIntRect buttonRect =
-    LayoutDeviceIntRect::FromUnknownRect(frame->GetScreenRect());
+    LayoutDeviceIntRect::FromAppUnitsToNearest(
+      frame->GetScreenRectInAppUnits(),
+      frame->PresContext()->AppUnitsPerDevPixel());
 
   // Get the widget rect in screen coordinates.
   nsIWidget *widget = GetNearestWidget();
   if (!widget) {
     aError.Throw(NS_ERROR_FAILURE);
     return;
   }
   LayoutDeviceIntRect widgetRect = widget->GetScreenBounds();