ISQL Version: LI-V2.5.2.26390 Firebird 2.5
Server version:
LI-V2.5.2.26390 Firebird 2.5

SESSION #1 // sysdba

[firebird@firebirdG empbuild]$ isql employee.fdb
Database: employee.fdb
SQL> create user usrxxx password '123'; commit;
SQL> create table supermain(id int, f01 int); commit;
SQL> insert into supermain values(1,100);
SQL> insert into supermain values(2,200);
SQL> insert into supermain values(3,300);
SQL> commit;
SQL> create role readmain;
SQL> commit;
SQL> grant select on supermain to readmain;
SQL> commit;
SQL> grant readmain to usrxxx;
SQL> commit;


SESSION #2 // user USRXXX, role with read_only right

[firebird@firebirdG empbuild]$ isql employee.fdb -user usrxxx -pas 123 -role readmain
Database: employee.fdb, User: usrxxx, Role: READMAIN
SQL> update supermain set f01=1;
Statement failed, SQLSTATE = 28000
no permission for update/write access to TABLE SUPERMAIN
SQL> rollback;
SQL> select * from supermain for update with lock; -- <<<<<<<<<<<<<<<<< passed! why ? <<<<<<<<<<<<

          ID F01
============ ============
           1 100
           2 200
           3 300


SESSION #1 // sysdba

SQL> set transaction read committed no record_version no wait;
SQL> select * from supermain;

          ID F01
============ ============
Statement failed, SQLSTATE = 40001
lock conflict on no wait transaction
-deadlock
SQL> rollback;
SQL> set transaction read committed no wait;
SQL> update supermain set f01=-f01 order by id rows 1;
Statement failed, SQLSTATE = 40001
lock conflict on no wait transaction
-deadlock
SQL> rollback;
SQL> set transaction read committed no wait;
SQL> delete from supermain;
Statement failed, SQLSTATE = 40001
lock conflict on no wait transaction
-deadlock