Here is an adaptation of the security database table USERS:

create domain rdb$user_name varchar(128) CHARACTER SET ASCII;
create domain rdb$uid integer;
create domain rdb$gid integer;
create domain rdb$password varchar(64);
create domain rdb$user_privilege integer;
create domain rdb$comment BLOB sub_type TEXT segment size 80
CHARACTER SET UNICODE_FSS;
create domain rdb$name_part varchar(128) CHARACTER SET UNICODE_FSS;

create table rdb$users(
rdb$user_name rdb$user_name not null primary key,
rdb$sys_user_name rdb$user_name,
rdb$group_name rdb$user_name,
rdb$uid rdb$uid,
rdb$gid rdb$gid,
rdb$password rdb$password,
rdb$privilege rdb$user_privilege,
rdb$comment rdb$comment,
rdb$first_name rdb$name_part,
rdb$middle_name rdb$name_part,
rdb$last_name rdb$name_part);

Question: are uid, guid, and group actually used for anything? They're
defined in access control lists (acls) from the days when users were
authenticated from /etc/passwd, but I don't see how they could be useful
in the current settings. If they are useful, this would be a very good
time to drop them. I'd much rather add them back if/when we know what
they mean than carry them around as lost baggage that might interfere
with future use.