Thanks, but your solution would only work for the Rectangle primitive tool.
I did manage to find a solution, by reading up about how to change rectangle corner radius in Fireworks. I tried drawing a rectangle and at the same time pressing the up and down arrow keys. Pressing up changes the inner radius so you end up creating a chamfer rectangle. Pressing down changes the outer radius, so you get a rounded rectangle. The more you press the key, the greater the change.
Hope this helps others who were getting annoyed with this problem.