At the height of its popularity, the i Pod made up almost 50% of Apple's revenue.
Now that the smartphones and tablets we use every day have the ability to store and play music, the i Pod's glory days are over.
i OS update files can be found at ~/Library/i Tunes, where "~" is your Home folder.
Notice, that software updates differ for different devices.
That means that, for instance, the i Pad firmware files will be kept at ~/Library/i Tunes/i Pad Software Updates, while you will find the same files for i Phone in ~/Library/i Tunes/i Phone Software Updates.
As i Phone users know, DFU mode is short for Development Firmware Upgrade, which means to upgrade or downgrade i Phone firmware manually.
If something goes wrong in the process of upgrading or downgrading your i Phone, you need to enter DFU mode to restore your i Phone or complete the downgrade process.