Mozilla wants to be bug free, and is putting some green behind the effort. Win a T-shirt and $500 US cash reward for reporting a valid “critical security bug” mozilla.org:

The Mozilla Security Bug Bounty Program is designed to encourage security research in Mozilla software and to reward those who help us create the safest Internet clients in existence. Reporters of valid critical security bugs will receive a $500 (US) cash reward and a Mozilla T-shirt.