Here is Saurik method to restore an old Firmware for the last models iPhone 3GS and iPod Touch with MC serial number, but since the last version of iTunes 9.0.2, it's impossible to restore a Firmware different than the last Firmware of Apple.
But, first of all, you need to save your ECID/ShSh on Saurik server when you were in Firmware 3.1.2

• For other models, you can restore with iREB and iTunes.

Note that Redsn0w is the only jailbreak tool for Firmware 3.1.3 for iPhone V1 and 3G only for the moment.

• Windows : go to Start and Run.
• Go to C:\Windows\System32\drivers\etc\hosts
• Pour Mac : /etc/hosts
• You must modify hosts file with Notepad.
• On the last line, add : 74.208.105.171 gs.apple.com or 74.208.10.249 gs.apple.com
• Save and close the file.

•  In Windows, Start and Run
• Type cmd.exe
• Then type :
  
  
  
  
  • ipconfig /flushdns
  • ipconfig /registerdns

• Go to DFU Mode on your iPhone.
• Shutdown your iPhone or iPod Touch.
• Button ON (3sec)
• Button ON + button Home (10sec)
• Button Home only (15 to 20sec, reboot)
• Start iTunes, it detects Recovery Mode.
• SHIFT+RESTORE
• Select the old Firmware 3.1.2 (on which your ECID/ShSh was saved in Cydia)

• If error 3002, you have not saved your ECID/ShSh.
• If error 1015, close iTunes and use irecovry Mac or Windows.

• Firmwares.