firebird-2.5.2.26539-1.mga2  is an upstream bug fix release
(http://tracker.firebirdsql.org/browse/CORE/fixforversion/10450)

example of bug fix upstream :http://tracker.firebirdsql.org/browse/CORE-3806

to test it under Mageia :

urpmi firebird-superserver

systemctl start firebird-superserver.service

$ isql-fb localhost:employee -user SYSDBA -password masterkey
Database:  localhost:employee, User: SYSDBA
SQL> create table t (col1 int, col2 int, col3 int);
SQL> insert into t values (100, 200, 300);
SQL> insert into t values (101, 201, 301);
SQL> insert into t values (102, 202, 302);
SQL> commit;
SQL> alter table t drop col1; 
SQL> select col2, col3 from t as t1 where exists (select * from t as t2 order by t1.col2 );

        COL2         COL3 
============ ============ 
         200          300 
         201          301 
         202          302 

SQL> commit;
SQL> drop table t;
SQL> exit;


before the update, the wrong result was :
SQL> select col2, col3 from t as t1 where exists (select * from t as t2 order by t1.col2 );

        COL2         COL3 
============ ============ 
         200          200 
         201          201 
         202          202 


Advisory
-----------------------
Upstream bugfix release that fix various bug and potentials server crash

(http://tracker.firebirdsql.org/browse/CORE/fixforversion/10333)
(http://www.firebirdsql.org/file/documentation/release_notes/html/rlsnotes252.html#bug-252)