--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -5205,16 +5205,30 @@ TransformBetweenRects(const gfxRect& aFr
 static nsRect
 TileNearRect(const nsRect& aAnyTile, const nsRect& aTargetRect)
 {
   nsPoint distance = aTargetRect.TopLeft() - aAnyTile.TopLeft();
   return aAnyTile + nsPoint(distance.x / aAnyTile.width * aAnyTile.width,
                             distance.y / aAnyTile.height * aAnyTile.height);
 }
 
+static gfxFloat
+StableRound(gfxFloat aValue)
+{
+  // Values slightly less than 0.5 should round up like 0.5 would; we're
+  // assuming they were meant to be 0.5.
+  return floor(aValue + 0.5001);
+}
+
+static gfxPoint
+StableRound(const gfxPoint& aPoint)
+{
+  return gfxPoint(StableRound(aPoint.x), StableRound(aPoint.y));
+}
+
 /**
  * Given a set of input parameters, compute certain output parameters
  * for drawing an image with the image snapping algorithm.
  * See https://wiki.mozilla.org/Gecko:Image_Snapping_and_Rendering
  *
  *  @see nsLayoutUtils::DrawImage() for the descriptions of input parameters
  */
 static SnappedImageDrawingParameters
@@ -5311,21 +5325,21 @@ ComputeSnappedImageDrawingParameters(gfx
     // Compute the anchor point in both device space and image space.  This
     // code assumes that pixel-based devices have one pixel per device unit!
     gfxPoint anchorPoint(gfxFloat(anchor.x)/aAppUnitsPerDevPixel,
                          gfxFloat(anchor.y)/aAppUnitsPerDevPixel);
     gfxPoint imageSpaceAnchorPoint =
       MapToFloatImagePixels(imageSize, devPixelDest, anchorPoint);
 
     if (didSnap) {
-      imageSpaceAnchorPoint.Round();
+      imageSpaceAnchorPoint = StableRound(imageSpaceAnchorPoint);
       anchorPoint = imageSpaceAnchorPoint;
       anchorPoint = MapToFloatUserPixels(imageSize, devPixelDest, anchorPoint);
       anchorPoint = currentMatrix.Transform(anchorPoint);
-      anchorPoint.Round();
+      anchorPoint = StableRound(anchorPoint);
     }
 
     gfxRect anchoredDestRect(anchorPoint, scaledDest);
     gfxRect anchoredImageRect(imageSpaceAnchorPoint, imageSize);
     transform = TransformBetweenRects(anchoredImageRect, anchoredDestRect);
     invTransform = TransformBetweenRects(anchoredDestRect, anchoredImageRect);
   }