Jailbreak iPhone 3G [Firmware 4.2.1]

Step 1:

On Mac:

1. Download the Mac version of Redsn0w
2. Open it and navigate to your Downloads folder, and find the folder called Redsn0w.
3. Open the Redsn0w folder and launch the app.

On Windows:

1. Download the Windows version of Redsn0w
2. Click open in the dialog box
3. Double click the Redsn0w folder
4. Double click on the Redsn0w application
5. Click run.

Step 2:

The next step is to download the appropriate firmware for your device. If you followed the rest of the guide, clicking on the download link will download the firmware that’s right for you. This download may take a bit of time.

Once you have your firmware, it’s time to jailbreak. Go to the Redsn0w window that you should already have open, and we can get started.

1. Click the browse button in the Redsn0w window, and select the firmware file you just downloaded.
2. Once you’ve selected your firmware, click next.
3. In the next screen there will be several checkboxes, but the crucial one to check is the “Install Cydia” checkbox.
4. Click next.

Step 3:

The next part of the process is to get your device ready for Redsn0w to do the jailbreak. It’s easy to mess this part up, so read through all of these steps to make sure you get it right. The instructions and a counter are right there in the app to help you out with this step after you click next. But make sure you are ready, because it starts right away. 

1. Turn off your device by holding the sleep button for 5 seconds and sliding to power off.
2. With the device turned off hold just the sleep button for 3 seconds.
3. After three seconds are up, continue holding the sleep button and then start holding the home button.
4. Hold the sleep and home buttons for 10 seconds. When 10 seconds pass, release the sleep button and continue holding the home button for 8 seconds.
5. After 8 seconds you can release the home button, and watch Redsn0w do it’s thing.

After about 5 minutes your device will reboot and you’ll see the Apple logo. If you can see the Cydia icon on your home screen, you’re jailbroken! Now you can start loading your device with tons of cool apps off the Cydia app store.

Downloads:

Redsn0w For Mac

Redsn0w For Windows

Firmware Download Link

SOURCE :GuideMyJailbreak