Thanks for all answers!
I'm still with this problem, I would like to revisit the issue.
I've checked transactions in mon$ tables and I can't find the transaction in order to kill it.
For example, right now I have:

Oldest transaction 963037
Oldest active transaction 1552161
Oldest snapshot 1552161
Next transaction 1552165

mon$transactions contents:

DB_KEY

MON$TRANSACTION_ID

MON$ATTACHMENT_ID

MON$STATE

MON$TIMESTAMP

MON$TOP_TRANSACTION

MON$OLDEST_TRANSACTION

MON$OLDEST_ACTIVE

MON$ISOLATION_MODE

MON$LOCK_TIMEOUT

MON$READ_ONLY

MON$AUTO_COMMIT

MON$AUTO_UNDO

MON$STAT_ID

00000023:00000001

1553643

377413

1

19.03.2014, 18:50:50.938

1553643

963037

mon$statments contents:

DB_KEY	MON$STATEMENT_ID	MON$ATTACHMENT_ID	MON$TRANSACTION_ID	MON$STATE	MON$TIMESTAMP	MON$SQL_TEXT	MON$STAT_ID
00000024:00000001	69013	377413	1554052	1	19.03.2014, 18:51:51.384	SELECT RDB$DB_KEY, MON$STATEMENT_ID, MON$ATTACHMENT_ID, MON$TRANSACTION_ID, MON$STATE, MON$TIMESTAMP, MON$SQL_TEXT, MON$STAT_ID FROM MON$STATEMENTS	5
00000024:00000002	53259	377413	NULL	0	NULL	SELECT RDB$DB_KEY, MON$TRANSACTION_ID, MON$ATTACHMENT_ID, MON$STATE, MON$TIMESTAMP, MON$TOP_TRANSACTION, MON$OLDEST_TRANSACTION, MON$OLDEST_ACTIVE, MON$ISOLATION_MODE, MON$LOCK_TIMEOUT, MON$READ_ONLY, MON$AUTO_COMMIT, MON$AUTO_UNDO, MON$STAT_ID FROM MON$TRANSACTIONS	6

I appreciate suggestions to find the transaction.
Thanks in advance!!

Vanina