Decode pre-2011 Apple Mac EFI/Firmware Password

Instructions

1. Boot up the Mac and log into an account with admin/root privileges
2. Open the Terminal app
3. Run the command

sudo nvram security-password
You should get output like:
security-password %fa%cb%d9%d9%dd%c5%d8%ce
4. Enter the code as displayed (without 'security-password ') into the Hash input text box below and press the Decode button

Decoder

Hash input
Plaintext output

Reference

Developed using the information at c|net's Use the Calculator to reveal a Mac's firmware password