The firewall setting is to not send a referrer variable, which is critical to spam prevention (as one can spoof a post to post to your forum without having to be posted from a valid location wihtout it)...
It can be more than jsut that, there is one thing to check before disabling the referrer check (if you want to nerf that security protection that is).
First, check to be sure ALL of your domains are in the allowed referrers:
CP -> Master Settings -> Primary Settings -> Advanced Options -> Domains for HTTP Referrer Check. Example:
http://www.undergroundnews.com|http://www.undergroundnews.net|http://www.undergroundnews.org|https://www.undergroundnews.com|https://www.undergroundnews.net|https://www.undergroundnews.org
That setting CAN cause the above error when a user is logged in and posting from the "non-www" version of your url when it isn't allowed in the check.
If you want to disable the check (which I would never recommend) then you can de-tick: "Disable HTTP Referer Check?"
The option in a firewall to block referrer variables isn't really a security setting, it's there to "protect privacy" by not forwarding referrers so that you can't be "traced" online, however the referrer check utilizes this variable to validate you're posting from an allowed host (as in one of the hosts which are in the allowed hosts box).
