You can only restore from a backup, not from the ipsw-firmware file. So, in case this folder is part of your computer backup routine, replace the whole folder and your latest backups should be listed again in iTunes.
Backups are stored in the backup folder, located here:
The folder where your backup data is stored changes depending on the computer's operating system. Since iTunes only keeps one backup per device, you should ensure the backup folder is included in your periodic data backup routine.
iTunes places the backup files in the following places:
- Mac: ~/Library/Application Support/MobileSync/Backup/
- Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
- Windows Vista and Windows 7: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
copied from http://support.apple.com/kb/HT1766
Message was edited by: Ingo2711