aah, I have had this problem in the past, all I do to fix it is to increase the shadow blur radius, I usually set mine to 3 all the time, and thus almost never see this problem, let me run some test renders and see what I can come up with.

this will take little while, as I am running poser on my computer at home, and controlling it via VNC.

as to why it happens, I think it has to do with the way firefly traces light. like most ray tracers it starts at the pixel on the image and traces the light back to the object and then from there to the light source. the problem is that it does this in "steps"...

TemplarGFX
3D Hobbyist since 1996
I use poser native units

167 Car Materials for Poser