--- a/gfx/layers/ipc/CompositorParent.cpp
+++ b/gfx/layers/ipc/CompositorParent.cpp
@@ -998,32 +998,32 @@ CompositorParent::TransformShadowTree(Ti
     treeTransform = gfx3DMatrix(ViewTransform(-scrollCompensation,
                                               gfxSize(mXScale, mYScale)));
 
     // If the contents can fit entirely within the widget area on a particular
     // dimenson, we need to translate and scale so that the fixed layers remain
     // within the page boundaries.
     if (mContentRect.width * tempScaleDiffX < metrics.mCompositionBounds.width) {
       offset.x = -metricsScrollOffset.x;
-      scaleDiff.height = NS_MIN(1.0f, metrics.mCompositionBounds.width / (float)mContentRect.width);
+      scaleDiff.width = NS_MIN(1.0f, metrics.mCompositionBounds.width / (float)mContentRect.width);
     } else {
       offset.x = clamped(mScrollOffset.x / tempScaleDiffX, (float)mContentRect.x,
                          mContentRect.XMost() - metrics.mCompositionBounds.width / tempScaleDiffX) -
                  metricsScrollOffset.x;
-      scaleDiff.height = tempScaleDiffX;
+      scaleDiff.width = tempScaleDiffX;
     }
 
     if (mContentRect.height * tempScaleDiffY < metrics.mCompositionBounds.height) {
       offset.y = -metricsScrollOffset.y;
-      scaleDiff.width = NS_MIN(1.0f, metrics.mCompositionBounds.height / (float)mContentRect.height);
+      scaleDiff.height = NS_MIN(1.0f, metrics.mCompositionBounds.height / (float)mContentRect.height);
     } else {
       offset.y = clamped(mScrollOffset.y / tempScaleDiffY, (float)mContentRect.y,
                          mContentRect.YMost() - metrics.mCompositionBounds.height / tempScaleDiffY) -
                  metricsScrollOffset.y;
-      scaleDiff.width = tempScaleDiffY;
+      scaleDiff.height = tempScaleDiffY;
     }
 
     // The transform already takes the resolution scale into account.  Since we
     // will apply the resolution scale again when computing the effective
     // transform, we must apply the inverse resolution scale here.
     gfx3DMatrix computedTransform = treeTransform * currentTransform;
     computedTransform.Scale(1.0f/container->GetPreXScale(),
                             1.0f/container->GetPreYScale(),