About iMac EFI Firmware Update 1.3 :

This update fixes several issues to improve the stability of iMac computers.

After the firmware is successfully applied to your Mac, your Boot ROM Version will be:

IM71.007A.B03

You can confirm the version of the BootROM installed on your computer using System Profiler.

To complete the firmware update process, please follow the instructions in the updater application (/Applications/Utilities/iMac EFI Firmware Update.app). The updater will launch automatically when the Installer closes.

For more information about firmware updates for Intel-based Macs, please visit: http://docs.info.apple.com/article.html?artnum=303364