"It will probably work in Poser 1 thru 4 because this is just a problem
specific to the Firefly renderer...."

Almost but not quite true (to be pedantic..)
In P5, using the double-sided square (or almost anything which has double-sided polys) with the P4 render can also cause artefacts. Different artefacts, and not so noticible, but there...
They show up as thin, horizontal, randomly-spaced dark lines....

Had me tearing (what remains of) my hair out for two days before I caught sight of a thread in this forum and got clued in.
Replaced all my double-sided squares (which I'd been using in my naivete 'cos I thought I was supposed to) with single-sided ones and the problem went away...

Cheers,
Diolma