Fire Password Viewer is a free program capable of discovering all the passwords you use in Mozilla Firefox: the password to your email account, Facebook, eBay, etc.
As you know, every time you enter a password on Mozilla Firefox (it's the same with other browsers too), it stores the password encrypted, to recover it in future sessions, if you grant permission. Well, Fire Password Viewer is able to reveal all those passwords.
The practicality of Fire Password Viewer is justified in case of a lost or forgotten password. However, it could also be used for constructive purposes, in cases of investigation. In any case, whichever way Fire Password Viewer is used, the user is accountable.