Thank you for your help! I couldn't even use Fireworks CS5 - just opening it gave me the internal error.
To fix, I closed Fireworks and
1) downloaded the CS5 updater http://www.adobe.com/support/fireworks/downloads_updaters.html
2) on Windows 7, renamed the two objects in folder C:\Users\[your name]\AppData\Roaming\Adobe\Fireworks CS5\English to add a suffix of OLD to each then re-opened Fireworks.
Object I renamed in C:\Users\[your name]\AppData\Roaming\Adobe\Fireworks CS5\English:
Fireworks CS5 Panel Prefs.xml to Fireworks CS5 Panel PrefsOLD.xml
Fireworks CS5 Preferences.txt to Fireworks CS5 PreferencesOLD.txt
Problem solved!