The Firebird user manager is an easy to use tool. Simply double click the User Manager node in the Enterprise Manager and the User Manager will open - alternatively, use the context menu Administration & Maintenance | User Manager.
You should have registered the server with a valid SYSDBA password in order to create users. If you're using a Classic architecture Firebird, be sure to have entered the security database path ("isc4.gdb" for Firebird < 1.5) in order to view the users. When using Database Workbench with the Classic architecture, you cannot create or delete users.
The Firebird User Manager
Selecting a user and using the Edit User button will open the User Properties dialog.
The User Properties Dialog
If you create a new user, this dialog will appear as well - but empty of course.
If you're modifying a user, and you don't want to modify the password - be sure to NOT change the current value in the Password and Confirm edit boxes.
Exitting this dialog by using the OK button will save the user information.