MYSQL: query does not use index for some time

Database of test:

SET NAMES utf8;
SET foreign_key_checks = 0;
SET time_zone = '+02:00';
SET sql_mode = 'NO_AUTO_VALUE_ON_ZERO';

CREATE TABLE `account` (
  `idAccount` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(128) NOT NULL,
  PRIMARY KEY (`idAccount`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8; 

CREATE TABLE `users` (
  `idUser` int(11) NOT NULL AUTO_INCREMENT,
  `idAccount` int(11) NOT NULL,
  `firstName` varchar(128) NOT NULL,
  PRIMARY KEY (`idUser`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

DROP TABLE IF EXISTS `transactions`;
CREATE TABLE `transactions` (
  `idTransactions` int(11) NOT NULL AUTO_INCREMENT,
  `idUser` int(11) NOT NULL,
  `dateTransaction` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`idTransactions`),
  KEY `index_dateTransaction` (`dateTransaction`) USING BTREE
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

INSERT INTO `transactions` (`idTransactions`, `idUser`, `dateTransaction`) VALUES
(1, 1,  '2012-12-16 15:52:32'),
(2, 1,  '2012-12-20 15:52:37'),
(3, 1,  '2013-02-01 15:52:37'),
(4, 2,  '2013-03-16 15:52:37'),
(5, 2,  '2013-03-18 15:52:37'),
(6, 3,  '2014-04-19 15:52:37'),
(7, 3,  '2014-05-20 15:52:37'),
(8, 4,  '2014-06-21 15:58:46');

INSERT INTO `account` (`idAccount`, `name`) VALUES
(1, 'Burger & Burger');

INSERT INTO `users` (`idUser`, `idAccount`, `firstName` ) VALUES
(1, 1,  'Roberto'),
(2, 1,  'Alessandro');

Depending with the date passed, sometimes MYSQL doesn't use the INDEX.

I know that I need to add / edit INDEX, please, could you please help me to perform this query very well?

This query doesn't use the INDEX:

SELECT
    users.firstName,
    ts1.*,
    COUNT(transactions.dateTransaction) AS num_transactions
FROM users
    INNER JOIN transactions ON transactions.idUser = users.idUser
    INNER JOIN (
        SELECT
            users.idUser,
            MIN(transactions.dateTransaction) AS first_transaction,
            MAX(transactions.dateTransaction) AS last_transaction
        FROM transactions
            INNER JOIN users ON transactions.idUser = users.idUser
        WHERE (users.idAccount = 1)
        GROUP BY users.idUser
    ) AS ts1 ON users.idUser = ts1.idUser
WHERE
    transactions.dateTransaction BETWEEN ('2012-01-01') AND ('2013-12-31')
AND users.idAccount = 1
GROUP BY users.idUser

EXPLAIN link: http://sqlfiddle.com/#!2/059d8/7/0

This query use it:

SELECT
    users.firstName,
    ts1.*,
    COUNT(transactions.dateTransaction) AS num_transactions
FROM users
        INNER JOIN transactions ON transactions.idUser = users.idUser
        INNER JOIN (
            SELECT
                users.idUser,
                MIN(transactions.dateTransaction) AS first_transaction,
                MAX(transactions.dateTransaction) AS last_transaction
            FROM transactions
                INNER JOIN users ON transactions.idUser = users.idUser
            WHERE users.idAccount = 1
            GROUP BY users.idUser
        ) AS ts1 ON users.idUser = ts1.idUser
WHERE
    transactions.dateTransaction BETWEEN ('2012-01-01') AND ('2012-12-31')
AND users.idAccount = 1
GROUP BY users.idUser

Change only the year.

But the biggest problem is that in production environment, with ~65.000 rows of transactions, query hangs on over 60 seconds (!)

I created a sqlfiddle, this is the link: http://sqlfiddle.com/#!2/059d8/1/0

Thank you very much!

---

Add the following two indexes:

ALTER TABLE `users` ADD KEY `bk1_account_user` (idAccount, idUser);

ALTER TABLE `transactions` KEY `bk2_user_datetrans` (idUser, dateTransaction);

This allows all the tables to be accessed by covering indexes, and eliminates some of the ALL type tables. See the SQLfiddle for details: http://sqlfiddle.com/#!2/b11bb/4

Also, consider upgrading to 5.6, to get rid of the "using join buffer".