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Open Firmware password protection for OS X prevents the computer from booting to alternative media, resetting parts of the memory, and using any of the many different startup options such as booting into single-user mode or safe boot mode unless the password is known. This helps increase security by preventing the current operating system from being bypassed at boot time or booting the computer into single-user mode gaining full access without any passwords (press Command and ’s’ during startup). Open Firmware requires that the password is entered to use the Startup Manager, which is accessed by pressing the Option key during startup.

Official Apple Open Firmware article with more details about the application here: http://support.apple.com/kb/HT1352

Install Open Firmware on OS X 10.5:

1. Insert the OS X 10.5 Disk 1
2. Click on the desktop or a finder window
3. Press Shift+Apple Key+g to open the “Go to Folder” window
4. Type or copy and paste in to the window: /Volumes/Mac OS X Install Disk 1/Applications/Utilities/
5. Click Go then drag and drop the Firmware Password Utility icon in to the Applications directory
6. Double click on the Firmware Password Utility icon in the Applications directory click Change then enter in a password.

Installation should be the same for OS X 10.4. Users of OS X 10.3 and below need to download the application from apple.com but should also upgrade OS X to the latest version for the additional security enhancements.