My new Poser 11 now works. This render used Firefly; the ground plane is set visible, and in its materials "Shadow Catch only" is set on, and as seen here, the ground plane appears in the preview, but not in the render, and the shadows on the ground plane appear on the grass as if the scene was photographed in one shot as seen here. OK, same as Poser 10.

But in Poser 11 in Superfly mode, the ground plane showed in full on the render.