I think this is less likely to be a bug and more likely to be a quirk in the new Mountain Lion OS, though it could certainly be related to the "eyedropper color sampling" bug you've observed previously.
Are you viewing the exported graphics in Preview?
According to an online source, untagged images are assumed to be in an sRGB color space in Mountain Lion. This is different from previous Mac OSs, which assumed the monitor display profile (a smaller gamut space, which required no color management). Graphics exported from Fireworks are untagged; therefore, they may be getting "assigned" an sRGB profile on-the-fly by the operating system. This would cause a change in the their appearance—including a boost in saturation, as sRGB is typically a larger-gamut color space than most monitor display profiles.
One way to test this theory would be to open the exported graphic in Preview and assign it the monitor display profile (go to Tools > Assign Profile..., then choose "Color LCD"); this should decrease the saturation of the graphic and cause it to appear as it does in Fireworks.
Also, this new Mac OS behavior should affect Fireworks PNGs as well, not just exported PNGs. Do you observe the same color change when you view a Fireworks PNG in Preview? How about graphics output from Fireworks CS5? They should also be affected.
Have you tried opening the graphic in Preview and assigning it the monitor display profile, as previously suggested? Or comparing the exported graphic to the Fireworks PNG (also within Preview)?
I'm curious about this issue but have no way of testing it myself. And I haven't found much actual documentation about changes in color management within Mountain Lion.
I'm wondering if exported graphics appear different in Preview, in a web browser, or both—in other words, in what contexts does the color appear to be wrong? If the graphic looks correct when viewed in a web browser, then it's probably just an annoying quirk with the operating system. But if the graphic looks wrong in a web browser, too... then it's more likely to be something that Adobe needs to address within the application.