It sounds like you have triggered a compatability mode. Find the Fireworks executable (probably buried under C:\Program Files\Adobe), right-click and check for compatibility. You may need to run in an XP-compatable mode, but that shouldn't mean you lose color depth, just the Aero theme.
No help. Fireworks did not block Aero in Vista,
cannot figure why it is in Windows 7
I have been experiencing the same issue in Bridge CS5. I was fine until I tried to batch rename files. As soon as the operation would start, it would kick me to Windows 7 Basic color scheme(I am running Windows 7 Pro 64 bit). After reading this post, I went to the start menu and right clicked on Bridge so I could access the properties window. I feel we should not have to run in compatability mode, isn't that why we upgrade? So I decided to check to see if it would work. I discovered that Bridge defaulted to XP Service Pack 3 compatibility mode which makes total sense since XP never did support the Aeroglass theme. After shutting off compatibility mode and restarting, everything is working fine.