How to restore iphone 3g custom ipsw once ipad baseband update

[first-born]

I have upgraded my baseband to 6.15 on 4.2.1. After that I attempted to downgrade to 3.1.2. It worked as well as my baseband was 6.15 on version 3.1.2. I attempted make use of Cydia for installing ultrasn0w however it remains crashing therefore I undertaken to restore back to a custom 4.2.1 ipsw. I made use of redsn0w to set the phone to DFU mode along with that used iTunes for restoring to the custom 4.2.1 ipsw. It returned to recovery screen once the restore was completed. I attempted to jailbreak it through redsn0w to obtain history of the recovery screen but it did not function. Can you tell me the correct method to restore to custom ipsw?

[Delaware]

You can try the following steps to solve the problem that you are facing. This worked for one of my friends, who was also having the same problem.

1. You have to make use of redsnow and click on firmware 4.2.1
2. Now mark in redsnow immediately enter pwned dfu mode
3. Your phone will show you black screen and this dfu mode
4. Now you have to restore through custom
5. This will let your phone to emergency mode
6. you have to use redsnow another time and mark just cydia option.
7. You will again find that your phone will hang at apple logo but don’t worry
8. Now enter boot theater.

This will make your phone to work. You have to use cydia and unlock your phone.

[Erie]

Follow this to solve your problems.

1. You should download iPhone 3GiPhone1,2_2.2.1_5H11_Custom_Restore.ipsw
2. Download the iPhone 3G iPhone1,2_2.2.1_5H11_Custom_Restore.ipsw from us (MD5 sum = 93c379cb80b573c41580bfb56619ff34). This leave out the baseband upgrade therefore you can be confident 2.2.1 will be well-matched by yellowsn0w unlock. It in addition holds Cydia and Installer. The logos for custom boot were barred.
3. Now attach your iPhone using USB cable as well as place it in DFU mode. To do that, turn off your phone. Hold power and home button jointly for about 10 seconds then free power but maintain holding home awaiting that computer beeps as the USB device is accepted. Display will come in no time.
4. Open iTunes and shift-click on Restore button. Then go to the custom restore 2.2.1 firmware that you have downloaded and iTunes will do the remaining work for you.

[Fox28]

I think that the firm ware that you have installed is not corrupted and that is why you are getting the problems like this. So my suggestion to you is to install the firmware again. I am saying you this because many of the times for the cases like this, I have seen that installing the latest firmware solves the problem and it can do the same for you.