ok. I solved! thank you guys!!
for everyone looking for the same solution I'll write my steps:
ps: I did these steps after I loaded and "empty" sketch to the board from a windows machine.
void setup(){}
void loop(){}
---------------------------------------------------------------------------------------------------------
INSTALL THE DFU PROGRAMMER (console commands)
---------------------------------------------------------------------------------------------------------
$ sudo aptitude install dfu-programmer
---------------------------------------------------------------------------------------------------------
DOWNLOAD AND VERIFY THE FIRMWARE (console commands)
---------------------------------------------------------------------------------------------------------
$ wget --no-check-certificate
https://github.com/arduino/Arduino/raw/master/hardware/arduino/firmwares/arduino-usbserial/Arduino-usbserial-uno.hex$ md5sum Arduino-usbserial-uno.hex
8e01ee236e70bbea43f7eb4e11c9688a Arduino-usbserial-uno.hex
---------------------------------------------------------------------------------------------------------
PUT ARDUINO UNO IN DFU MODE
---------------------------------------------------------------------------------------------------------
0) connect your board to the pc
1) hold the first wire on the top left 8u2 ICSP pad
2) touch the second wire against the left side of the capacitor below the RX LED (leds will blink)
3) remove the second wire
4) remove the first wire
---------------------------------------------------------------------------------------------------------
UPLOAD THE NEW FIRMWARE (console commands)
---------------------------------------------------------------------------------------------------------
$ sudo dfu-programmer at90usb82 erase
$ sudo dfu-programmer at90usb82 flash Arduino-usbserial-uno.hex
$ sudo dfu-programmer at90usb82 reset
---------------------------------------------------------------------------------------------------------
FINAL STEP
---------------------------------------------------------------------------------------------------------
disconnect the board from the pc
reconnect it and have fun
