I don't know if this helps, but this is what I do (btw, I'm a Firebird
newbie).

I use gsec and isql and import my commands from a test file.

# gsec code to create a user
gsec -database 192.168.1.100:/opt/firebird/security.fdb -user sysdba
-password masterkey -a myuser -pw mypassword

# text file for isql import
# log in (my example is connecting to Firebird on a Linux server)
CONNECT "192.168.1.100:/opt/firebird/db/donordb.fdb" user 'SYSDBA'
password 'masterkey';

# create my tables (no need to show that code)

# create a ROLE and grant permissions to tables
CREATE ROLE MY_USER;
GRANT ALL ON TABLE1 TO ROLE MY_USER;
GRANT ALL ON TABLE2 TO ROLE MY_USER;
GRANT ALL ON TABLE3 TO ROLE MY_USER;

# assign role to user account
GRANT MY_USER TO myuser;

This is not an advanced setup as I will be doing all my security via
my application interface.

I hope this helps.