Mac Firmware Passwords
Setting a firmware password in OS X prevents your Mac from starting up from any device other than the built in hard drive. Locking your Mac from Find My Mac also sets a firmware password that you’ll then need to enter in order to use your Mac.
If you don’t set a firmware password, someone can boot the Mac into Recovery Mode and reset the password on your main partition. This would allow access to all unencrypted (i.e. non-Keychain) data. However, the firmware password isn’t that secure because Apple can bypass it. Plus, the drive could be physically removed from your Mac and attached to a different Mac. So, if you want to prevent access to your data, it’s better to use FileVault. The firmware password only prevents your Mac from being booted without access to your data, which probably doesn’t matter much.
