This seems a security breach waiting to happen: the password search tool finds credentials by partial content of passwords.
Visually, it appears like showing and copying a password is guarded by entry of the master password. But quite clearly, the search lets us figure out whether or not someone is using a password at all.
This is a problem in situations where a user has logged into Firefox, already provided their master password, and walks away from their desk for a couple of minutes. (This happens regularly with many of my clients. People never lock their desktops. They'll even go home without locking their desktop, so maintenance personell has full access to their computers and networks.)