Execute the following command to retrieve your display and allow X11 clients to connect back to your X11 display that made the SSH connection into the remote host running the X11 client. eg: firefox
xauth add $(xauth -f ~YOURUSERNAME/.Xauthority list|tail -1)
Where YOURUSERNAME is the username of your command line or shell or whatever which ran the ssh -X connection to the remote host.
Run the above xauth command after sudo’ing to root (or other target user).
FYI,
Richard.