Apache and Postfix
I'm new to pfSense and would like to ask your opinion about using this os as a web/mail server.
I don't want to open ports on my wireless router, but from the internet side apache/postfix should be reachable.
My idea was that:
Internet - Firewall - Router - My PC
That way I don't need another computer and the servers are 'totally' out of my network.
What do you think?
I don't think you'll find anyone here who will advocate running pfSense as a web or mail server.
It is a cut-down install of FreeBSD, many of the components you would need are not present and would require installing. By the time you have installed all the software you need to do this you will likely have broken pfSense in some fundamental way or at the very least reduced it's security below what is acceptable for a firewall.
People have done it in the past however and there are some long complex threads about it here. IMHO, it would be far better to use an OS designed to offer all-in-one, like Zentyal or SME Server, or install FreeBSD and configure it to your liking.
A good compromise, if you want to run pfSense and I'm in favour of that ;), would be to run the box as a VM host with pfSense and your servers as VMs. There are many users running that setup.