The Firefly renderer was not original designed for ray tracing or something. I don't really understand the excuses. I just know that you can never trust it to show reflections of a displaced surface. Sometimes it will work, sometimes not. The probability of not working goes up as the amount of displacement increases, and the size of the displacedment features, as a fraction of a polygon's total area, decreases. Some internal math formula is making bad assumptions somewhere.

Tweaking Raybias helps. Decreasing displacement helps. Making smaller/more polygons helps.

SInce I'm not into modeling, I did the first two of those three.

Changed Raybias to 12 inches.
Decreased your displacement amount to .7 inch (you had about .98 inch).

Artifacts gone. (mostly - still a teensy bit at the top - but not really easy to see)

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)