User name XYZ could not be used for SQL role
You might get this error when trying to create a new role. Since roles and usernames share the same namespace, you cannot have a role with the same name as some user. The tricky part of this problem is that Firebird does not store usernames in databases, so it checks the privileges table to see if privileges for such user exists (and then concludes that such user does exist). If you are sure that such user does not exist anymore, revoke all his privileges before creating the role. To find out what are those privileges, you can query the RDB$USER_PRIVILEGES table:
WHERE RDB$USER = 'new_role_name'