Vc tentou isso:
# cd /opt/firebird/bin
# ./changeDBAPassword.sh
Please enter current password for SYSDBA user : masterkey
Please enter new password for SYSDBA user : biroguin
GSEC> GSEC>
Running ed to modify /etc/init.d/firebird
#
http://www.firebirdsql.org/manual/pt_br/fbutils-scripts-dba-password-pt_br