I have to admit that I don’t really use Fireworks that much these days, as Illustrator and Photoshop more than meet my design requirements, but Fireworks does have one powerful ace up its sleeve. If you want to use PNGs with alpha transparency, then unfortunately, Photoshop will only allow you to export 24-bit PNGs (PNG-24), which are heftier than 8-bit PNGs.
Using Fireworks, we can save in 8-bit PNG, even when the image requires alpha transparency. In my experience, this can remove 20% or more off the file size with little to no loss in quality.
All you have to do is open up your 24-bit PNG, then, in the PNG compression settings panel in Fireworks, set it to 8-bit with alpha transparency, and then export. Voila, you have much smaller images to use!
To learn more about using PNGs in web designs, see the Web Designer’s Guide to PNG Image Format.