I need Firmata so I can control my Arduino directly from PC, not just manual button pressing.
No, you don't. No-one ever needs Firmata. If you explain what you are sending from your PC, and what you want the Arduino to do with what it gets sent, we can probably help you modify your sketch.
I need to insert Firmata inside my sketch in order to communicate from Arduino to PC and vice versa.
You can't do that. You can run your sketch, or you can run Firmata. As has been explained, Firmata turns your Arduino into a dumb slave device.