The b3 version of redsn0w fixes a problem where re-running redsn0w over an existing jailbreak would cause MobileSubstrate-based apps to stop running until MS was installed again. Now you can re-run the redsn0w jailbreak step without worrying about that (but still remember to de-select the “Install Cydia” option if it’s already installed).
If auto-detection fails and redsn0w tells you no identifying data was found, you can always pre-select the appropriate 5.0.1 IPSW
using “Extras->Select IPSW”.
Here are the redsn0w download links: