diff --git a/layout/base/FrameLayerBuilder.cpp b/layout/base/FrameLayerBuilder.cpp
index e20b04edc916..3563960d24be 100644
--- a/layout/base/FrameLayerBuilder.cpp
+++ b/layout/base/FrameLayerBuilder.cpp
@@ -667,7 +667,8 @@ struct MaskLayerUserData : public LayerUserData
     return mRoundedClipRects == aOther.mRoundedClipRects &&
            mScaleX == aOther.mScaleX &&
            mScaleY == aOther.mScaleY &&
-           mOffset == aOther.mOffset;
+           mOffset == aOther.mOffset &&
+           mAppUnitsPerDevPixel == aOther.mAppUnitsPerDevPixel;
   }
 
   nsRefPtr<const MaskLayerImageCache::MaskLayerImageKey> mImageKey;
@@ -678,6 +679,7 @@ struct MaskLayerUserData : public LayerUserData
   float mScaleX, mScaleY;
   // The ContainerParameters offset which is applied to the mask's transform.
   nsIntPoint mOffset;
+  int32_t mAppUnitsPerDevPixel;
 };
 
 /**
@@ -3414,6 +3416,7 @@ ContainerState::SetupMaskLayer(Layer *aLayer, const DisplayItemClip& aClip,
   newData.mScaleX = mParameters.mXScale;
   newData.mScaleY = mParameters.mYScale;
   newData.mOffset = mParameters.mOffset;
+  newData.mAppUnitsPerDevPixel = mContainerFrame->PresContext()->AppUnitsPerDevPixel();
 
   if (*userData == newData) {
     aLayer->SetMaskLayer(maskLayer);
@@ -3422,8 +3425,8 @@ ContainerState::SetupMaskLayer(Layer *aLayer, const DisplayItemClip& aClip,
   }
 
   // calculate a more precise bounding rect
-  const int32_t A2D = mContainerFrame->PresContext()->AppUnitsPerDevPixel();
-  gfxRect boundingRect = CalculateBounds(newData.mRoundedClipRects, A2D);
+  gfxRect boundingRect = CalculateBounds(newData.mRoundedClipRects,
+                                         newData.mAppUnitsPerDevPixel);
   boundingRect.Scale(mParameters.mXScale, mParameters.mYScale);
 
   uint32_t maxSize = mManager->GetMaxTextureSize();
@@ -3479,7 +3482,10 @@ ContainerState::SetupMaskLayer(Layer *aLayer, const DisplayItemClip& aClip,
 
     // paint the clipping rects with alpha to create the mask
     context->SetColor(gfxRGBA(1, 1, 1, 1));
-    aClip.DrawRoundedRectsTo(context, A2D, 0, aRoundedRectClipCount);
+    aClip.DrawRoundedRectsTo(context,
+                             newData.mAppUnitsPerDevPixel,
+                             0,
+                             aRoundedRectClipCount);
 
     // build the image and container
     container = aLayer->Manager()->CreateImageContainer();