diff --git a/gfx/thebes/gfxUtils.cpp b/gfx/thebes/gfxUtils.cpp
index a7962daf0..3567412a6 100644
--- a/gfx/thebes/gfxUtils.cpp
+++ b/gfx/thebes/gfxUtils.cpp
@@ -758,7 +758,7 @@ gfxUtils::ClipToRegion(DrawTarget* aTarget, const nsIntRegion& aRegion)
 }
 
 /*static*/ gfxFloat
-gfxUtils::ClampToScaleFactor(gfxFloat aVal)
+gfxUtils::ClampToScaleFactor(gfxFloat aVal, bool aRoundDown)
 {
   // Arbitary scale factor limitation. We can increase this
   // for better scaling performance at the cost of worse
@@ -779,14 +779,17 @@ gfxUtils::ClampToScaleFactor(gfxFloat aVal)
 
   gfxFloat power = log(aVal)/log(kScaleResolution);
 
-  // If power is within 1e-5 of an integer, round to nearest to
-  // prevent floating point errors, otherwise round up to the
-  // next integer value.
   if (fabs(power - NS_round(power)) < 1e-5) {
+    // If power is within 1e-5 of an integer, round to nearest to
+    // prevent floating point errors.
     power = NS_round(power);
-  } else if (inverse) {
+  } else if (inverse != aRoundDown) {
+    // Use floor when we are either inverted or rounding down, but
+    // not both.
     power = floor(power);
   } else {
+    // Otherwise, ceil when we are not inverted and not rounding
+    // down, or we are inverted and rounding down.
     power = ceil(power);
   }
 
diff --git a/gfx/thebes/gfxUtils.h b/gfx/thebes/gfxUtils.h
index 4619570c8..fbb47e4b3 100644
--- a/gfx/thebes/gfxUtils.h
+++ b/gfx/thebes/gfxUtils.h
@@ -128,9 +128,10 @@ public:
 
     /**
      * Return the smallest power of kScaleResolution (2) greater than or equal to
-     * aVal.
+     * aVal. If aRoundDown is specified, the power of 2 will be less than
+     * or equal to aVal.
      */
-    static gfxFloat ClampToScaleFactor(gfxFloat aVal);
+    static gfxFloat ClampToScaleFactor(gfxFloat aVal, bool aRoundDown = false);
 
     /**
      * Clears surface to aColor (which defaults to transparent black).
diff --git a/layout/painting/FrameLayerBuilder.cpp b/layout/painting/FrameLayerBuilder.cpp
index 9509f2ea2..0a3a851e0 100644
--- a/layout/painting/FrameLayerBuilder.cpp
+++ b/layout/painting/FrameLayerBuilder.cpp
@@ -5247,6 +5247,23 @@ static void RestrictScaleToMaxLayerSize(gfxSize& aScale,
     aScale.height /= scale;
   }
 }
+
+static nsSize
+ComputeDesiredDisplaySizeForAnimation(nsIFrame* aContainerFrame)
+{
+  // Use the size of the nearest widget as the maximum size.  This
+  // is important since it might be a popup that is bigger than the
+  // pres context's size.
+  nsPresContext* presContext = aContainerFrame->PresContext();
+  nsIWidget* widget = aContainerFrame->GetNearestWidget();
+  if (widget) {
+    return LayoutDevicePixel::ToAppUnits(widget->GetClientSize(),
+                                         presContext->AppUnitsPerDevPixel());
+  } else {
+    return presContext->GetVisibleArea().Size();
+  }
+}
+
 static bool
 ChooseScaleAndSetTransform(FrameLayerBuilder* aLayerBuilder,
                            nsDisplayListBuilder* aDisplayListBuilder,
@@ -5311,20 +5328,7 @@ ChooseScaleAndSetTransform(FrameLayerBuilder* aLayerBuilder,
         aContainerItem->GetType() == nsDisplayItem::TYPE_TRANSFORM &&
         EffectCompositor::HasAnimationsForCompositor(
           aContainerFrame, eCSSProperty_transform)) {
-      // Use the size of the nearest widget as the maximum size.  This
-      // is important since it might be a popup that is bigger than the
-      // pres context's size.
-      nsPresContext* presContext = aContainerFrame->PresContext();
-      nsIWidget* widget = aContainerFrame->GetNearestWidget();
-      nsSize displaySize;
-      if (widget) {
-        LayoutDeviceIntSize widgetSize = widget->GetClientSize();
-        int32_t p2a = presContext->AppUnitsPerDevPixel();
-        displaySize.width = NSIntPixelsToAppUnits(widgetSize.width, p2a);
-        displaySize.height = NSIntPixelsToAppUnits(widgetSize.height, p2a);
-      } else {
-        displaySize = presContext->GetVisibleArea().Size();
-      }
+      nsSize displaySize = ComputeDesiredDisplaySizeForAnimation(aContainerFrame);
       // compute scale using the animation on the container (ignoring
       // its ancestors)
       scale = nsLayoutUtils::ComputeSuitableScaleForAnimation(
@@ -5361,6 +5365,19 @@ ChooseScaleAndSetTransform(FrameLayerBuilder* aLayerBuilder,
         if (clamp) {
           scale.width = gfxUtils::ClampToScaleFactor(scale.width);
           scale.height = gfxUtils::ClampToScaleFactor(scale.height);
+
+          // Limit animated scale factors to not grow excessively beyond the display size.
+          nsSize maxScale(4, 4);
+          if (!aVisibleRect.IsEmpty()) {
+            nsSize displaySize = ComputeDesiredDisplaySizeForAnimation(aContainerFrame);
+            maxScale = Max(maxScale, displaySize / aVisibleRect.Size());
+          }
+          if (scale.width > maxScale.width) {
+            scale.width = gfxUtils::ClampToScaleFactor(maxScale.width, true);
+          }
+          if (scale.height > maxScale.height) {
+            scale.height = gfxUtils::ClampToScaleFactor(maxScale.height, true);
+          }
         }
       } else {
         // XXX Do we need to move nearly-integer values to integers here?