leyba@leyba:~$ sudo chmod -R 777 /home/leyba/db_dir
leyba@leyba:~$ ls -l
...
drwxrwxrwx 2 firebird firebird     4096 мар 23 10:34 db_dir
...

leyba@leyba:~$ grep firebird /etc/group
firebird:x:126:leyba

leyba@leyba:~$ sudo isql-fb
Use CONNECT or CREATE DATABASE to specify a database
SQL> CREATE DATABASE 'localhost:/home/leyba/db_dir/test_db.fdb' page_size 16384
CON> user 'USER1' password '1';

Statement failed, SQLSTATE = 28000
no permission for CREATE access to DATABASE /home/leyba/db_dir/test_db.fdb
SQL> 

Use CONNECT or CREATE DATABASE to specify a database
SQL>  CREATE DATABASE 'localhost:/home/leyba/db_dir/test_db.fdb' page_size 16384 
CON> user 'SYSDBA' password 'masterkey';
SQL> connect "localhost:/home/leyba/db_dir/test_db.fdb" user 'SYSDBA' password 'masterkey';
Database: "localhost:/home/leyba/db_dir/test_db.fdb", User: SYSDBA
SQL> grant create database to USER1;
Statement failed, SQLSTATE = 0A000
unsuccessful metadata update
-GRANT failed
-feature is not supported
-Only grants to USER or ROLE are supported for CREATE DATABASE
SQL> SELECT RDB$GET_CONTEXT('SYSTEM','ENGINE_VERSION') FROM RDB$DATABASE;

RDB$GET_CONTEXT                                
============== 
3.0.1     

...
SQL> grant create database to user USER1;
SQL> quit;
leyba@leyba:~$ isql-fb
Use CONNECT or CREATE DATABASE to specify a database
SQL>  CREATE DATABASE 'localhost:/home/leyba/db_dir/user1_db.fdb' page_size 16384
CON> user 'USER1' password '1';
SQL>