Sn0wbreeze is still a mess, and its "author" is a jerk who stole Dev Team code without permission or credit and made it proprietary in violation of the GPL. His original version also contained copyrighted Apple code, endangering the entire jailbreaking community by giving Apple a leg to stand on with its jailbreaking = piracy arguments. iH8sn0w should be ashamed of himself, and anyone who appreciates free (as in speech) software and the huge service the Dev Team has done for us should stay well away from Sn0wbreeze.
Also? Snowbreeze will not jailbreak 3.1.3, it only allows you to upgrades a jailbroken phone to 3.1.3 without losing its jailbreak. There is no tool which will jailbreak a 3GS upgraded to stock 3.1.3 firmware.