Does Firebird support field-level access rights?

Yes, it does for writing new values (UPDATE statements). To control the rights, use the GRANT and REVOKE statements:

GRANT UPDATE(field1,field2,...) ON table1 TO USER1;
REVOKE UPDATE(field1,field2,...) ON table2 FROM USER2;

If you wish to limit users to certain fields when reading (SELECT), a common way is to use views:

create view v1 (limited column list)
as
select limited,column,list
from t1;

And then grant user SELECT rights only for the view. With views, you can also limit which records (rows) can user see:

create view v1 (column,list)
as
select column,list
from t1
where ...constraining clause...;


If you need really complex rules, you can setup up a stored procedure that would return NULLs for some columns to specific users.