I wish I found out about this a while ago. Apparently in Photoshop you can optimize a png. Not sure about fireworks?
Anyway in Photoshop go to Image->Adjustment->Posterize.
A slider will then popup, adjust the slider and then save. Be careful it removes colors to reduce the size so look carefully before you save it.
You can read more about it here:
http://www.queness.com/post/2507/most-effective-method-to-reduce-and-optimize-png-images