diff --git a/gfx/layers/d3d9/ContainerLayerD3D9.cpp b/gfx/layers/d3d9/ContainerLayerD3D9.cpp
index 2671ea0514f6..cada63361367 100644
--- a/gfx/layers/d3d9/ContainerLayerD3D9.cpp
+++ b/gfx/layers/d3d9/ContainerLayerD3D9.cpp
@@ -240,6 +240,8 @@ ContainerLayerD3D9::RenderLayer()
 #endif
     GetEffectiveTransform().Is2D(&contTransform);
     NS_ASSERTION(is2d, "Transform must be 2D");
+    mSupportsComponentAlphaChildren = (GetContentFlags() & CONTENT_OPAQUE) ||
+        (mParent && mParent->SupportsComponentAlphaChildren());
   }
 
   /*