For anyone who wants to set transparency using Adobe Fireworks:
Under "Optimize" (on the right side of the screen if you're using the default workspace), select PNG 8 and then "Alpha Transparency" (use PNG 24 if it's a 24-bit PNG with no transparency, or PNG 32 if it's a 24-bit PNG with semi-transparency or transparency):
Then just File > Export.
If you're looking for something more Photoshop-esque, File > Image Preview is what you're looking for.