diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index 95ccce192a8..9555564c4fe 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -1247,30 +1247,16 @@ nsCSSRendering::PaintBoxShadowOuter(nsPresContext* aPresContext,
       shadowContext->NewPath();
       if (hasBorderRadius) {
         gfxCornerSizes clipRectRadii;
-        gfxFloat spreadDistance = -shadowItem->mSpread / twipsPerPixel;
-        gfxFloat borderSizes[4] = { 0, 0, 0, 0 };
+        gfxFloat spreadDistance = shadowItem->mSpread / twipsPerPixel;
 
-        // We only give the spread radius to corners with a radius on them, otherwise we'll
-        // give a rounded shadow corner to a frame corner with 0 border radius, should
-        // the author use non-uniform border radii sizes (border-top-left-radius etc)
-        // (bug 514670)
-        if (borderRadii[C_TL].width > 0 || borderRadii[C_BL].width > 0) {
-          borderSizes[NS_SIDE_LEFT] = spreadDistance;
-        }
+        gfxFloat borderSizes[4];
 
-        if (borderRadii[C_TL].height > 0 || borderRadii[C_TR].height > 0) {
-          borderSizes[NS_SIDE_TOP] = spreadDistance;
-        }
+        borderSizes[NS_SIDE_LEFT] = spreadDistance;
+        borderSizes[NS_SIDE_TOP] = spreadDistance;
+        borderSizes[NS_SIDE_RIGHT] = spreadDistance;
+        borderSizes[NS_SIDE_BOTTOM] = spreadDistance;
 
-        if (borderRadii[C_TR].width > 0 || borderRadii[C_BR].width > 0) {
-          borderSizes[NS_SIDE_RIGHT] = spreadDistance;
-        }
-
-        if (borderRadii[C_BL].height > 0 || borderRadii[C_BR].height > 0) {
-          borderSizes[NS_SIDE_BOTTOM] = spreadDistance;
-        }
-
-        nsCSSBorderRenderer::ComputeInnerRadii(borderRadii, borderSizes,
+        nsCSSBorderRenderer::ComputeOuterRadii(borderRadii, borderSizes,
             &clipRectRadii);
         shadowContext->RoundedRectangle(shadowGfxRect, clipRectRadii);
       } else {
diff --git a/layout/base/nsCSSRenderingBorders.cpp b/layout/base/nsCSSRenderingBorders.cpp
index 1470ef73c7b..b266b2b52dd 100644
--- a/layout/base/nsCSSRenderingBorders.cpp
+++ b/layout/base/nsCSSRenderingBorders.cpp
@@ -221,6 +221,38 @@ nsCSSBorderRenderer::ComputeInnerRadii(const gfxCornerSizes& aRadii,
   iRadii[C_BL].height = NS_MAX(0.0, aRadii[C_BL].height - aBorderSizes[NS_SIDE_BOTTOM]);
 }
 
+/* static */ void
+nsCSSBorderRenderer::ComputeOuterRadii(const gfxCornerSizes& aRadii,
+                                       const gfxFloat *aBorderSizes,
+                                       gfxCornerSizes *aOuterRadiiRet)
+{
+  gfxCornerSizes& oRadii = *aOuterRadiiRet;
+
+  // default all corners to sharp corners
+  oRadii = gfxCornerSizes(0.0);
+
+  // round the edges that have radii > 0.0 to start with
+  if (aRadii[C_TL].width > 0.0 && aRadii[C_TL].height > 0.0) {
+    oRadii[C_TL].width = NS_MAX(0.0, aRadii[C_TL].width + aBorderSizes[NS_SIDE_LEFT]);
+    oRadii[C_TL].height = NS_MAX(0.0, aRadii[C_TL].height + aBorderSizes[NS_SIDE_TOP]);
+  }
+
+  if (aRadii[C_TR].width > 0.0 && aRadii[C_TR].height > 0.0) {
+    oRadii[C_TR].width = NS_MAX(0.0, aRadii[C_TR].width + aBorderSizes[NS_SIDE_RIGHT]);
+    oRadii[C_TR].height = NS_MAX(0.0, aRadii[C_TR].height + aBorderSizes[NS_SIDE_TOP]);
+  }
+
+  if (aRadii[C_BR].width > 0.0 && aRadii[C_BR].height > 0.0) {
+    oRadii[C_BR].width = NS_MAX(0.0, aRadii[C_BR].width + aBorderSizes[NS_SIDE_RIGHT]);
+    oRadii[C_BR].height = NS_MAX(0.0, aRadii[C_BR].height + aBorderSizes[NS_SIDE_BOTTOM]);
+  }
+
+  if (aRadii[C_BL].width > 0.0 && aRadii[C_BL].height > 0.0) {
+    oRadii[C_BL].width = NS_MAX(0.0, aRadii[C_BL].width + aBorderSizes[NS_SIDE_LEFT]);
+    oRadii[C_BL].height = NS_MAX(0.0, aRadii[C_BL].height + aBorderSizes[NS_SIDE_BOTTOM]);
+  }
+}
+
 /*static*/ void
 ComputeBorderCornerDimensions(const gfxRect& aOuterRect,
                               const gfxRect& aInnerRect,
diff --git a/layout/base/nsCSSRenderingBorders.h b/layout/base/nsCSSRenderingBorders.h
index 648a660dfcf..f35a3a7820e 100644
--- a/layout/base/nsCSSRenderingBorders.h
+++ b/layout/base/nsCSSRenderingBorders.h
@@ -236,6 +236,14 @@ struct nsCSSBorderRenderer {
   static void ComputeInnerRadii(const gfxCornerSizes& aRadii,
                                 const gfxFloat *aBorderSizes,
                                 gfxCornerSizes *aInnerRadiiRet);
+
+  // Given aRadii as the border radii for a rectangle, compute the
+  // appropriate radii for another rectangle *outside* that rectangle
+  // by increasing the radii, except keeping sharp corners sharp.
+  // Used for spread box-shadows
+  static void ComputeOuterRadii(const gfxCornerSizes& aRadii,
+                                const gfxFloat *aBorderSizes,
+                                gfxCornerSizes *aOuterRadiiRet);
 };
 
 #ifdef DEBUG_NEW_BORDERS
diff --git a/layout/reftests/box-shadow/boxshadow-threecorners-ref.html b/layout/reftests/box-shadow/boxshadow-threecorners-ref.html
new file mode 100644
index 00000000000..cd9f383e1f8
--- /dev/null
+++ b/layout/reftests/box-shadow/boxshadow-threecorners-ref.html
@@ -0,0 +1,3 @@
+<!DOCTYPE HTML>
+<div style="position: absolute; top: 300px; left: 70px; width: 360px; height: 160px; border-bottom-left-radius: 40px; border-top-left-radius: 40px; border-top-right-radius: 40px; background: red;">&nbsp;</div>
+<div style="position: absolute; top: 30px; left: 100px; width: 300px; height: 100px; border-bottom-left-radius: 10px; border-top-left-radius: 10px; border-top-right-radius: 10px; background: yellow;">Test</div>
diff --git a/layout/reftests/box-shadow/boxshadow-threecorners.html b/layout/reftests/box-shadow/boxshadow-threecorners.html
new file mode 100644
index 00000000000..f00135f9597
--- /dev/null
+++ b/layout/reftests/box-shadow/boxshadow-threecorners.html
@@ -0,0 +1,2 @@
+<!DOCTYPE HTML>
+<div style="position: absolute; top: 30px; left: 100px; width: 300px; height: 100px; border-bottom-left-radius: 10px; border-top-left-radius: 10px; border-top-right-radius: 10px; box-shadow: 0 300px 0 30px red; background: yellow;">Test</div>
diff --git a/layout/reftests/box-shadow/boxshadow-twocorners-ref.html b/layout/reftests/box-shadow/boxshadow-twocorners-ref.html
new file mode 100644
index 00000000000..bc3263cdb17
--- /dev/null
+++ b/layout/reftests/box-shadow/boxshadow-twocorners-ref.html
@@ -0,0 +1,3 @@
+<!DOCTYPE HTML>
+<div style="position: absolute; top: 300px; left: 70px; width: 360px; height: 160px; border-bottom-left-radius: 40px; border-top-right-radius: 40px; background: red;">&nbsp;</div>
+<div style="position: absolute; top: 30px; left: 100px; width: 300px; height: 100px; border-bottom-left-radius: 10px; border-top-right-radius: 10px; background: yellow;">Test</div>
diff --git a/layout/reftests/box-shadow/boxshadow-twocorners.html b/layout/reftests/box-shadow/boxshadow-twocorners.html
new file mode 100644
index 00000000000..e62c316f2b6
--- /dev/null
+++ b/layout/reftests/box-shadow/boxshadow-twocorners.html
@@ -0,0 +1,2 @@
+<!DOCTYPE HTML>
+<div style="position: absolute; top: 30px; left: 100px; width: 300px; height: 100px; border-bottom-left-radius: 10px; border-top-right-radius: 10px; box-shadow: 0 300px 0 30px red; background: yellow;">Test</div>
diff --git a/layout/reftests/box-shadow/reftest.list b/layout/reftests/box-shadow/reftest.list
index abe0be23547..5289bf29cd5 100644
--- a/layout/reftests/box-shadow/reftest.list
+++ b/layout/reftests/box-shadow/reftest.list
@@ -16,6 +16,8 @@ random-if(layersGPUAccelerated) == boxshadow-mixed.html boxshadow-mixed-ref.html
 random-if(d2d) == boxshadow-rounded-spread.html boxshadow-rounded-spread-ref.html
 HTTP(..) == boxshadow-dynamic.xul boxshadow-dynamic-ref.xul
 random-if(d2d) == boxshadow-onecorner.html boxshadow-onecorner-ref.html
+random-if(d2d) == boxshadow-twocorners.html boxshadow-twocorners-ref.html
+random-if(d2d) == boxshadow-threecorners.html boxshadow-threecorners-ref.html
 == boxshadow-skiprect.html boxshadow-skiprect-ref.html
 
 == overflow-not-scrollable-1.html overflow-not-scrollable-1-ref.html