Hi Dawid,
THANKS A LOT for take your time answering my question. 
I understood that the SYN Flood protection is used to protect the NETWORK against DOS attacks and the SYN-ACK-ACK protect the firewall self (and as consequence protect the network against DOS).
So, lets imagine that a syn flood will be initiate, the SRX should intercept the SYN's and will respond the SYN's in behalf of the server, right? So, i can suppose that the syn shouldn't never reach the server due the tcp-proxy-feature, right? So, why i use the syn-flood screen option althought the fact that the syn-ack-ack can be enough?
The TCP-Proxy feature is enabled by default? Or not?
Tks,
João Victor