Hi,
when user come to my site to register and have their firewall turned on they cant register...why? and how can i solve this? And then site will be more unsecure?
Thanks in advance!

Software Firewalls usually block the HTTP Referrer, which tells your site, where the user is coming from. If your site doesn't receive this referrer, it will not allow changes to the database (e.g. inserts and updates) but only select statements.

The check is called in include/common.php, where you can just remove that check. The security level enhancement from this step can be questioned, but many small steps...

There are SO many threads about this thing

Try this thread.