Observation; Raytrace mesh artifacts as above must be specific Firefly bug as I've yet to see similar in other Raytrace renderers.

Yes, I'm using Firefly (Poser 9). I see the mesh-line artifacts regardless of whether Smooth Polys is checked. Bizarrely, adjusting shadow min bias has the opposite effect (i.e. increasing it makes them bigger, decreasing it makes them smaller), but only for the mesh-line artifacts on the back-lit mesh. Mesh-line shadow artifacts on other objects behave exactly as you say.  :oS Definitely seems to be a back-lit + one-sided-mesh + Firefly + ray-traced-shadows specific problem

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.