Whenever someone tries to upgrade their device and then jailbreak the same, the jailbreaking tools like PwnageTool would ask you to download and keep the bootrom file ready because it would be required while jailbreaking the device.
We can easily find the iPhone device current OS version by visiting the Settings > General > About which gives you the detailed information. But the bootrom information is not given which needs to be found using a application called iDetector , a tiny application from the iH8sn0w team which had earlier released the Sn0wbreeze tool.
The application once downloaded will ask you to connect your device and put the 3GS into DFU Mode.
Once its into the DFU Mode, the utility will display the status of the iPhone in the form of a quick report which can be used to find the correct boot loader file.
