Fireworks places strokes around all sides of an object by default. There are a couple of workarounds you can try. One is to clone the object, offset it by a pixel or two in the direction you want the stroke, change the color of the clone to the color you want for the strioke, and then move the clone behind the original object. You can also use a drop shadow of two or three pixels, rotated to the side you want the outline on, set the shadow opacity to 100%, the softness to 0, and change its color from the default black to whichever color you wish.
The easiest solution is probably to set the shape to fill only, clone the shape, set the clone to stroke only and then remove the sides for which you don't want a stroke using the knife tool.