Investigating more closely, I noticed that if I just viewed the image in Firefox (that is, I pointed my browser directly at the PNG file), I could see that where the top and bottoms touched, the pixels were blurred. That suggested to me that there was something strange about the way Firefox was rendering images. A Google search lead me to this image-rendering [developer.mozilla.org] page. I added this to the styles and it worked:
Note, this is marked as an experimental technology, so you may want to use the vender prefixed versions as described on that page.