Typically the iMac Firmware Updater will place a copy of itself in the "Startup Items" folder before it updates the firmware. Upon restarting, the update will launch again. If the firmware has been updated, the following message will appear.
Figure 1: Dialog box noting iMac firmware update.
After clicking OK, the application will automatically move itself to the trash.
The behavior you describe results from the application not being moved to the trash. In that case, the dialog will appear every time the computer is restarted.
Evidently, there is a file or folder named "iMac Firmware Updater" in the trash.
Solution
To make the dialog quit appearing, simply empty the trash or remove the file named "iMac Firmware Updater" from the Startup Items folder.