To handle extreme shaped polys like in this case, you need extreme render settings as well. Try this:

 - in render settings / manual settings, set Min Shading Rate to something very small. At most 0.1 but 0.01 might even be better.
Why: Firefly turns each poly into subpolys, and assigns colors and displacements etc to each of them, based on the underlying texture maps etc. The size of each subpoly is such that each covers at least a ... portion of a single pixel in your render. So when ... is a small value you'll get a lot of subpolys. The other way around: when you want such a small triangle split up well, and have a busy texture like you have assigned properly, you need a lot of such subpolys. So, how so set that ... value? Simple: that's min Shading Rate.
Your Auto-Settings in the middle set MSR to 1.0, that's 1 subpoly per render pixel. You need 10 to 100, so 0.1 to 0.01 are the preferred values.

 - set pixel samples to at least 3 (at most 10).
Why: both are related: Min Shading Rate should be slightly smaller than 1/ (Pixel Samples)^2 so PS = 3 makes MSR < 1/(33) = 0.11 while PS=10 makes MSR< 1/(1010) = 0.01

 - set post filter size to 3, and filter type to sinc.
Why: 1 means NO filter, 4 is to extreme, 2 is soft (female skin), 3 is hard (male skin, wall structure, ...) and sinc is the best balanced of all.

I don't know of it really helps, but that would be my first approach. I'd like to know the effect.

- - - - - 

Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though