What version of Poser are you using?

If it's Poser 6, set your render options manually, rather than using the automatic settings.  Lowering the shading rate, increasing pixel samples, and adjusting the max. texture size can make a big difference.

Also, save your image as a TIFF or other uncompressed file type.   The default is really low-res JPG, which usually looks terrible. 

This FAQ has a lot of great info on getting the most out of  Firefly:

http://www.keindesign.de/stefan/poser/firefaq.html