There are different versions of Open Firmware but give this combination a try:
Boot into Open Firmware by holding down the 'Command-Option-O-F' keys from a cold start while the chime is being played. You will see a command-line screen...enter these commands:
1. At the Open Firmware prompt, type: reset-nvram
2. Press Return.
3. At the Open Firmware prompt, type: reset-all
4. Press Return.
Example:
0 > reset-nvram
Press Return
0 > reset-all
Press Return
The reset-all command should cause the computer to restart. If this occurs, you have successfully reset the Open Firmware settings.