Highest firmware a 3GS can be restored to?

[ann713]

Hi all, I haven't done this in a while so I appreciate any help. I need to restore an 8GB 3GS back to a firmware that will still permit be to unlock and run an untethered jailbreak, believe it's currently unlocked on 4.2.1 right now. What is the highest firmware I can custom restore back to?

 

[Joell]

if you've backed up your SHSHs through cydia you can restore to anyone of those
restoring to iOS 5 is still tethered so be cautious