> isql -user sysdba employee

  SQL> create user SYSDBA password 'mast3rkey';
  SQL> commit;
  SQL> quit;

CREATE OR ALTER USER SYSDBA PASSWORD <новыйпароль>

isql -q -z -user sysdba security.db

firebird.conf:
  UserManager = Srp, Legacy_UserManager

isql:
alter user sysdba set password 'ПАРОЛ' using plugin Legacy_UserManager;

del security?.fdb
isql -q -z
  create 'security.db' user 'sysdba' ...
  create user sysdba set password 'ПАРОЛ';
  exit;