I tried the following test:

# cd /opt/firebird/bin
# isql -user sysdba - password masterkey
# CONNECT /opt/firebird/examples/employee.fdb;

I receive the following message:

Statement failed, SQLCODE = -902

Your to user name and password ploughs not defined. Administrator Ask your
database you set up the Firebird login.

I ask as to configure such login?

your installation will have generated a SYSDBA passwrod for you. Read the
release notes and look in the install dir
Alan