Related questions:
How to downgrade iPhone 2G/3G firmware 3.1.3 to 3.1.2, 3.1, etc.?
How to fix after jailbreak no carrier/no signal problem?
How to jailbreak/unlock iPhone while on firmware 3.1.3?
How to downgrade your baseband?
Fact: You can't jailbreak firmware 3.1.3 yet, please stay tuned for the updates.
If you have upgraded your firmware to version 3.1.3 then you have to downgrade your firmware to an other version (like, 3.1.2, 3.1, etc.) before you could jailbreak your iPhone 2G/3G.
How to downgrade your iPhone 2G/3G?
How to downgrade iPhone 2G/3G firmware 3.1.3 to 3.1.2, 3.1, etc.?
Follow the following steps:
Step 1: Download an older release from here. I recommend 3.1 or 3.1.2.
Step 2: Connect the device to your computer and backup all your data through the latest version of iTunes (in case something goes wrong).
Step 3: Select the device on your iTunes. Then click on the Restore button while pressing the Shift (or Option) button of your computer.
Step 4: A file chooser window will pop up, select the older firmware you've downloaded (e.g., iPhone1,2_3.1.2_7D11_Restore.ipsw, iPhone1,2_3.1_7C144_Restore.ipsw, etc.). This will start restoring your firmware but, here is the sweet deal, using the older ipsw your have just selected.
Now just wait till your iTunes pops up an error message (10/15 minutes or so). Don't worry about the message, iTunes has already installed the old firmware on your device. Now your you will find that your iPhone is in "recovery mode". It's normal, don't panic.
Step 5: Download blackra1n from here, run it. This will jailbreak your iPhone (wait while blackra1n reboots your device).
If blackra1n isn't working for you (it happens, especially on windows vista and 7 x64) then right click on blackea1an, go to Properties > Compatibility. Check Run this program in compatibility mode for: Windows XP (Service Pack 3); and Privilege Level: Run this program as an administration. Press Apply and try running the tool again.
Wonderful! You've successfully jailbroken your iPhone 2G/3G!
However, you may find that your device can't find your carrier's signal and showing message: "NO CARRIER" or "NO SIGNAL". This is because your device is still using the new baseband for firmware 3.1.3 (ICE2- 05.12.01).
How to fix after jailbreak "NO CARRIER" problem?
How to downgrade your baseband to ICE2- 04.26.08?
Step 1: Install Cydia and sn0w using blackra1n that you've just installed on your iPhone (look above, Step-5).
Step 2: Search in cydia for fuzzyband, downlaod it and run it to downgrade your baseband to an older version (e.g., ICE2- 04.26.08).
Step 3: Go into cydia > Manage > Source and press the Edit button. An Add buttion will appear on the left top corner, press that and type repo666.ultrasn0w.com in the URL field on the pop up window. Hit Add Source. This will add repo666.ultrasn0w.com in the souces, install that.
Step 4: Reboot your device.
Done!
