When I add field in view (use ALTER VIEW), in parallel connections there is an error.
Use 2 windows isql.

1. In FIRST isql:

SET SQL DIALECT 3;
create database 'localhost:d:\Program Files\Firebird\bases\test_view.fdb' user 'SYSDBA' password 'masterke' page_size 16384;
create table t(a integer, b integer, c integer);
commit;
insert into t values(1,2,3);
commit;
create view v(a, b) as
select a, b from t;
commit;
select * from v;

           A B
============ ============
           1 2


2. In SECOND isql:

connect 'localhost:d:\Program Files\Firebird\bases\test_view.fdb' user 'SYSDBA' password 'masterke';
select * from v;

           A B
============ ============
           1 2
commit;


3. In FIRST isql:

commit;
alter view v(a, b, c) as
select a, b, c from t;
commit;


4. In SECOND isql:
select * from v;

           A B C
============ ============ ============
Statement failed, SQLSTATE = 22000
no current record for fetch operation