I've seen this happen in several models, and even occasionally in my own work, so I spent a lot of time trying to track it down. What I discovered is that this is not really a Poser or Firefly problem, but can happen in many types of render engines. In fact, it happens in UVMapper, so I have learned to use that program as I model to check my work (just be sure that the "Render Backfaces" option is turned OFF). See the attached screen-cap from UVMapper which is two simple boxes which intersect and overlap each other - it shows the blackened surfaces effect and the hashed shading where surfaces overlap. The actual cause of this is two overlapping surfaces in EXACTLY the same plane when one of the surfaces has normals (more accurately winding order) reversed with respect to the other surface. Fixing this in Poser is not always easy or even consistent because it depends on how the surfaces are grouped and materialized. If the surfaces BOTH are in the same group AND have the same material applied, there is NO way to have it render properly in Poser5. If the two surfaces have DIFFERENT materials, then using very slight displacement on ONE of the materials will provide enough separation between them to have them render. Using the Alternate Diffuse method might help render the black surfaces which are NOT overlapping, but will do nothing for the hashmarks on the ocerlapping areas. The best solution is to avoid overlapping surfaces as a model is being made.