and what i've found so far:

Firefly doesn't respect Blender smoothing groups, even though you can see them in Poser's preview.  it does respect hard creases from Blender.  so if you select an edge, Mark Sharp (under Ctrl-E or the Edge menu), and add an Edge Split modifier, you can get good edges.  i'm having a totally separate weird problem with smoothing right now, but i have some hope for figuring out something  with Blender.

RubiconDigital's method is more robust because it gives you variable smoothing across the item (smooth 30 here, smooth 50 there, etc.).  but the edge selecting method gives you at least some control. 

you can also now export polygroups in Blender OBJs, so you can create smoothing groups that way, too.

exporting "Edges" creates a weird break in the mesh.  i don't quite understand it, but it is interesting.