I solved the problem myself.
All my configuration was correct, there is a bug with the squid addon: you can not start the squid service from the web interface, when you click on "start the service" in the image below nothing happens, the service doesn't start and you don't have any error message:
0_1533074663644_squid screenshot.png
I had to connect to the pfsense terminal and run a "ps aux |grep squid" to see that th service was not running (i didn't have any error message in /var/log/squid).
A simple "squid start" solves the problem, does someont knows where can i report this bug ?