-- Создаём тестового пльзователя
CREATE USER TEST PASSWORD 'test';

-- Роль для него
CREATE ROLE TEST_ROLE;

-- Добавляем тэг в которм прописана роль по умолчанию
ALTER USER TEST TAGS(DEFAULT_ROLE = 'TEST_ROLE');

-- Назначаем роль пользователю
GRANT TEST_ROLE TO TEST;

SET TERM^;

-- Ну и собствено триггер на событие соединения с БД
CREATE OR ALTER TRIGGER TR_ON_CONNECT
ACTIVE ON CONNECT POSITION 0
AS
BEGIN
  FOR
      SELECT
          SEC$VALUE AS DEFAULT_ROLE
      FROM
          SEC$USER_ATTRIBUTES
      WHERE SEC$KEY = 'DEFAULT_ROLE'
        AND SEC$USER_NAME = CURRENT_USER AS CURSOR C
  DO
  BEGIN
    EXECUTE STATEMENT 'SET ROLE ' || C.DEFAULT_ROLE;
  END
END^

SET TERM;^

Use CONNECT or CREATE DATABASE to specify a database
SQL> connect 'localhost:test' user 'test' password 'test';
Database:  'localhost:test', User: test
SQL> select current_role from rdb$database;

ROLE
===============================
TEST_ROLE

SQL>

CREATE OR ALTER TRIGGER TR_ON_CONNECT
ACTIVE ON CONNECT POSITION 0
AS
BEGIN
  IF (CURRENT_ROLE = 'NONE') THEN
  BEGIN
    FOR
        SELECT
            SEC$VALUE AS DEFAULT_ROLE
        FROM
            SEC$USER_ATTRIBUTES
        WHERE SEC$KEY = 'DEFAULT_ROLE'
          AND SEC$USER_NAME = CURRENT_USER AS CURSOR C
    DO
    BEGIN
      EXECUTE STATEMENT 'SET ROLE ' || C.DEFAULT_ROLE;
    END
  END
END