Ssh -X user@router X11Forwarding
on pfSense 2.3.4-RELEASE-p1 (amd64) we installed X11Forwarding with:
1.) pkg add http://pkg.freebsd.org/freebsd:10:x86:64/latest/All/xauth-1.0.10.txz
then add all demanded packages
2.) then xauth is installed under
3.) in file /etc/sshd change / add
(with menu "Diagnostics/ edit file")
$sshconf .= "X11Forwarding no\n";
$sshconf .= "XAuthLocation /usr/local/bin/xauth \n";
4.) in file /etc/ssh/ssh_config
# ForwardX11 no
# XAUthLocation /usr/local/bin/xauth
While you are free to do that on your own, it's an awful idea. Don't load X libraries on the firewall for convenience, especially using a non-pfSense package repository. The odds are high that you'll end up causing yourself problems down the road trying to maintain that system.
Don't use the firewall for SSH X forwarding, use a VPN and hit what you need directly.