I don't see any ngons in the wireframe.

The problem appears to be entirely due to smooth shading and not because of the geometry. This is a known issue for Poser and modelers creating for Poser MUST either model directly for it, including chamfers and additional edges so that smooth shading calcs don't distort limited geometry, or they must adjust the smooth shading effects for their objects, manually, once inside of Poser.

It is more desirable, by far, to model specifically for Poser and to take that into account when modeling low-poly inorganic objects, specifically, even those that may have some smooth transformation in their topology. That means that, for Poser-specific models, models will generally have more edges/faces than necessary for renderers other than Firefly, which is specifically targeted towards rendering organic shapes very well, but inorganic ones (like boxes, flat objects, sharp-edged objects, not so well.

There used to be a submission guide around here that concerned the standards that .obj models must adhere to. In short - Chamfered edges, all quads, no ngons, limited stars/diamonds, etc.. are the norm. At least, what "should" be paid attention to in an object modeled for use in Poser/Firefly.

It's worth doing a google to see, exactly, how smooth shading works so that you can create models that will not exhibit smooth shading artifacts under normal rendering conditions. When modeling for Poser and when in doubt, the solution is always "more polys... " :) (Especially near sharp edges, so the shading calcs have enough info and aren't limited by single calcs across such a large distance.)

PS - Instead of chamfering those edges, which isn't desirable in this model, you can use what some refer to as "guide edges", specifically added so that smooth shading algorithms have enough verts to calc to render a hard edge that is not distorted. (Add another two edges, very close to the "sharp edges", for instance, and you may also wish to add additional edges down the length of the model so you don't get smooth-shading "buckling" at certain spots where calcs become useless, since there's no new geometry to correct them.) Granted, however, it is difficult to get Firefly to render hard edges without doing what would normally be considered as over-inflating the geometry of a particular model. BUT, if one wants something that works just fine with smooth shading, right out of the box, without additional user fiddling-with for users of Poser/Firefly, this is just what you have to do.