Here's how you can set a firmware password for your Mac. This might help protect your privacy if someone tries to access the data on your Mac by booting off an external drive.
1. Restart your Mac and hold the Command and R keys down while restarting.
2. The OS X Utilities menu will appear.
3. Click on Utilities.
4. Click on Firmware Password Utility.
5. Click on the Turn On Firmware Password.
6. Enter your new firmware password, then click the Set Password button.
7. Click on Quit Firmware Password Utility.
8. Restart your Mac by clicking on the apple symbol in the menu and choosing Restart.
Want to keep your roommate from posting your nude selfies on Facebook? Concerned that your Mac might fall into the wrong hands? You've already got a secure password on your Mac, maybe even file encryption. What happens if someone boots off an external hard drive?
Setting a firmware password is the solution. It comes with a few consequences and drawbacks, however. Follow along to read about the risks associated with firmware passwords as well as a guide on how to set one for those that think it's still worth it.More at iMore
Additional resources for OS X