Will choosing a Workspace Layout take care of this? These are available from the control bar at the top of the interface on the right side (Expanded Mode, Iconic Mode, etc.) or from the main menu, under Window > Workspace Layouts.
If this doesn't work, you could remove or delete the Panel Preferences file for Fireworks. On a Mac, this is found under User > Library > Preferences > Adobe Fireworks CS6 > en > Fireworks CS6 Panel Prefs.xml. (Not sure where this file is located on Windows, but this should give you an idea.) Quit Fireworks before you remove this file. Then when you restart, the file will regenerate itself using the default settings.