--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -4136,22 +4136,22 @@ nsContextBoxBlur::Init(const nsRect& aRe
   if (aRect.IsEmpty()) {
     mContext = nsnull;
     return nsnull;
   }
 
   gfxFloat scaleX = 1;
   gfxFloat scaleY = 1;
 
-  // Do blurs in device space when possible
-  // If the scale is not uniform we fall back to transforming on paint.
+  // Do blurs in device space when possible.
   // Chrome/Skia always does the blurs in device space
   // and will sometimes get incorrect results (e.g. rotated blurs)
   gfxMatrix transform = aDestinationCtx->CurrentMatrix();
-  if (transform.HasNonAxisAlignedTransform()) {
+  // XXX: we could probably handle negative scales but for now it's easier just to fallback
+  if (transform.HasNonAxisAlignedTransform() || transform.xx <= 0.0 || transform.yy <= 0.0) {
     transform = gfxMatrix();
   } else {
     scaleX = transform.xx;
     scaleY = transform.yy;
   }
 
   // compute a large or smaller blur radius
   gfxIntSize blurRadius = ComputeBlurRadius(aBlurRadius, aAppUnitsPerDevPixel, scaleX, scaleY);