There is a field RDB$COMMENT in RDB$USERS.
Accessing that field in security2.fdb is no more possible.
To compensate for it, a number of user management commands was implemented: http://www.firebirdfaq.org/faq19/
I wish the users description also could be queried by SQL with commands like
COMMENT ON USER name IS 'text'
and/or ALTER USER name SET COMMENT 'text'
It might help browsing user list in client apps (for example setting user grants when taking users from LDAP/ActiveDirectory) , it might help distinguish users created for different DBs by different APPs in inherited environments
For FB3 that probably may need security plugins API review, then the earlier the better