Just came across this same issue in CS5. Sometimes when you resize an image using the "Image size" function, Fireworks does not properly scale either the width or the height to the closest absolute pixel. This creates a sub pixel which becomes apparent with exporting GIFs. You can see the scaling issue if you throw a 1 pixel solid hard edge border around your image: if you see a transparent gap on the outside of the border then you've got the issue & it's this that causes the GIF image to go blurry when exported. The fix is to crop the image changing the canvas size to "absolutely" crop the image.
Generally the animated Fireworks GIFs exports are a buggy part of an otherwise great web graphic design tool: