It's an old problem that has (perhaps) been worse after SR1. I't sbeen a while since I've seen people complaining about it, but after SR1, you're not the first to do so.

Basically it's caused by double sided polygons. Try checking Normals_Forward on the materials. If that doesn't help, try adding a noise node as displacement at a very low setting. Like 0.002 or so. Too small to notice, but large enough to "pull the vertices apart" so to speak.

The Poser 4 renderer isn't a raytracer and does't care AT ALL about normals, which is why you won't see it there. 

In short: Firefly dislikes two-sided polygons :)

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.