It doesn't look quite like polygonal self shadowing, a side effect of Reyes rendering exacerbated by either too low a shadow min bias, or Poser phong smoothing, or AO.

You can try turning off smoothing in Firefly render settings, raising your shadow min bias in properties of your raytrace shadow light, and or applying subdivision to the mesh(es) in lieu of smoothing. Usually a level of subdivision helps in conjunction with smoothing off. Back in the day before SubD in Poser I would fiddle with shadow min bias and arrived at 0.350 as a value that helped. Values below 0.200 are a sure culprit if you are using smoothing (which was really necessary back then to eliminate the faceted look of many models)...

But caveat, like I said it doesn't look quite like the artifact type I described... which could best be described as a sort of mesh XRay appearing over the surface of the afflicted model when rendered. A bit hard to tell in your image.