My use case: I made a screenshot of a user interface which contained a button on white background. After pasting this into Fireworks, I want to select this button together with a tiny shadow it creates on the white background. Is there a tool like "Smart Marquee" which would allow me to roughly select the area with the button and then would shrink automatically so that only the smallest area required is selected? Doing this manually using Marguee selection tool is error prone as I may miss some very light shades of the shadow (not to say that it's quite inconvenient).
Can a lasso or magic wand tool be used for that? If so, how?
You can adjust the sensitivity of the Magic wand tool, so, theoretically, you could do what you want with it. However, you would have to waste so much time fine tuning it that it wouldn't be worth it. What I do is zoom in so I can see the colors at pixel level, and then make my selection with the Polygon Lasso tool.
Can you select the white background, then invert the selection?
If necessary, you can then clean up the edges using the lasso tool along with Shift and Ctrl-Shift. (I think those are the keys, anyway, my fingers are usually on automatic, now!)