If you are saving your .png file, then you get a file that has data in it specific to Fireworks. (It's allowed by the PNG specification.) This will have alpha channel information and layer information in it.
If you want a "normal" .png image, then you want to export, not save. When you export, you have the choice between 32-bit (full color + alpha transparency), 24-bit (full color, no transparency, and 8-bit (indexed color, colors can be semi-transparent).
I think what you want to do, then, is export your Fireworks document as a 24-bit .png. I like using File > Image Preview and selecting the image type and exporting from there.