Same as your other post, this has nothing to do with color management. You should re-post in the Fireworks forum.
This question is related to color management, but it's specific to application color management instead of a full color management workflow.
This will be a very difficult question for anyone not on the Fireworks team itself to answer, because I believe Fireworks is an unmanaged application. It doesn't have a color management system like Photoshop, so it would be very difficult to compare or characterize what it's doing, unless you performed a tremendous amount of trial and error.
Blending mode behavior has nothing to do with color management. This has to do with a Fireworks feature and how it behaves differently (in terms of calculations) than a similar Photoshop feature. This should be posted in the Fireworks forum.