never used fireworks, but with PS, turn on your grids, (VIEW-->SHOW-->GRID) then using the rectangular marquee tool, drag a rectangle out to the size of your border using the grid as a ref. (Helps to keep your border constant). Then inverse the selection (CTRL SHIFT I) and on a new layer fill with black.

If you want your border *not* to cover any of the existing image, then you can expand the canvas size (IMAGE-->CANVAS SIZE), and then create a new layer behind your image and fill with black. This method though will increase the overall size of your image just so you know :

~Vp~