Pulled up Anton's hair because it's so versatile & did renders of 3 different styles - long, short & up.

In the long, you can see where firefly seems to be reading the texture map wrong - but it's fine in the P4 render.
In the short, both images seem to be similar - no obvious texure problems in the firefly render.
In the updo, it appears to be okay except in the top wisps (firefly) - P4 render is fine.

And yes, backfacing polygons is/was checked