Hi, I’m Gayan from Sri Lanka. I bought an iMac 2013. The firmware is lock and I need to unlock it and I don’t seem to remember the password. Can you help me with this?
See the discussion at http://forums.macrumors.com/showthread.php?t=1573579 . Several methods are suggested there.
You can try the 'old' method which is used on older models (<2010)
1. shutdown and unplug the iMac
2. remove one of the memory modules (see the manual about removing it, it's not hard)
3. plug the power cable back into the iMac
4. start up and perform a double PRAM reset (3 dongs)
you can do so by press and holding the "Command + Option + P + R" keys
5. now turn it off again and unplug
6. put the memory back in, replug and start up normally
However, as initially stated this is the method for old models. There is a good chance that Apple increased the security and it is no longer working.
For the time being the only confirmed 'solution' is to bring the iMac to an Apple Authorized Service Provider (or Apple directly) with a proof of purchase. They will then remove the firmware lock.