Here are some tutorials on making bitmap selections in Fireworks:
In addition to using bitmap tools—like Marquee, Lasso, Polygon Lasso, etc.—to create a selection, you can use a vector tool like the Pen tool to create a vector shape, then convert that shape to a selection using the Modify > Convert Path to Marquee command.
Thanks for help. :-)
I have found a way to fix the problem
Also, if you're trying to cut something from a consistent flat or linear gradient background, like in a screenshot, this extension can help automatically fill the background in from where you cut the image: http://johndunning.com/fireworks/about/FillWithBackground