i d like for example to check button/sensors states on processing
Firmata is not essential for that to happen.
do stuff on computer,
Firmata is not essential for that to happen.
and then send back to arduino (via serial) some values, to print on LCD.
Firmata is not essential for that to happen.
Or connecet an I2C sensor, add its library
Firmata is not essential for that to happen.
As Grumpy Mike said, if the Arduino is to be more than a dumb device, like a keyboard or mouse, you MUST ditch Firmata.
So i thing that i could mix codes in firmata but without serial.
The Processing application talks to the Firmata sketch using the serial port, so, no, you can't.
Also i tried to avoid firmata and send via serial an array to processing but i got HUGE delay
Well, you haven't posted any code for either end, so we have no idea. Plenty of guesses, but they won't help you.
I searched about and everyone suggested firmata
Obviously your search did not include the Arduino site. Nobody here recommends it (unless you ignore the nots sprinkled all over the place).
which is perfect and got no delay
It isn't perfect or you would not be complaining about its limitations.
but i think that this is an important issue for people that are not programmers (like me)
How are you writing a program in Processing, then? Wishful thinking will only get you so far. At some point, you need to quit wishing and start learning to be a programmer.