I think the problem was with texttures created for the Poser 4 renderer. Some would have visible seams rendered in Firefly, unless you set the shading rate very low. Or turned texture filtering on, but that made things look blurry. Probably not an issue these days, when nobody's making textures for the P4 rendererer any more.

I might have to give up on this shader. Raising the shading rate looks fine at a distance, but for a closeup, you lose the glittery look, unless you set the shading rate to 10 or something. Maybe I'll just Photoshop it.