Worked for me...
I have a jailbroken iphone 4 with a tethered setup.
Safari and a few other things began crashing, but I think it's because I let the battery die out on my tethered jailbroken phone
Anyway:
I opened "redsn0w_win_0.9.15b3" which i initially used to jailbreak my iphone to os6. When I first jailbroke, I had to use this same process.
I selected "extras" and then "manually specify an ipsw".
You have to find and download the software version that works for this jailbreak. For me it was "iPhone3,1_6.0_10A403_Restore" Download it and save it.
Anyway...locate and select this file.
This should make your phone reboot with the old OS
Then in redsnow, I selected "just boot" and followed instructions.
once rebooted, everything worked fine.
DISCLAIMER: Not a tech guy, first time giving advice. Just spent too many hours searching and trying overly difficult solutions to simple problems...hopefully didn't mess anyone up and this helps. If not, apologies.