well, I updated just the Arduino-usbserial-mega.hex, to make everything work.
The date reported on the file is 21 may 2011.
The link is exactly the one showed here (the same indicated in the forum):
https://github.com/arduino/Arduino/tree/master/hardware/arduino/firmwares
Note that I downloaded all the folders clicking the "Downolads" button in the upper left corner
of the internet page. This download the file "arduino-Arduino-fcb66dd.tar".
Open it and go to Hardware/arduino/firmwares/arduino-usbserial. There there is the hex file.
bye!