Fireworks allows the user to create a vector shape that can be used as an image mask for any object or objects that are to be grouped with the mask. This is useful for creating gradients that have transparent features that are going to be composited with other objects in Fireworks. When using objects as masks in Fireworks it is important to remember that black pixels in the mask will make the masked objects opaque and white pixels in the mask will make the masked objects transparent. This is because the mask works as an Alpha when the Modify > Mask Group > Mask to Image command is used.
In order to preserve the Alpha channel on export, use the PNG32 format and set the Matte color to Transparent in the Optimize panel. This will allow other image editing programs to open the file with the Alpha channel intact.