X11Forwarding must be set to allow the action on the site you are SSHing toSSH server (in your case the Ubuntu box) and you should setin its ForwardXllsshd_config in your client config on, and you must allow X11 to be forwarded for the other sideSSH client (your Fedora box). This can be done in by passing the default options-X option or your user ssh configediting the ssh_config file to add the ForwardX11 default.
Fixing typo of "XLL" to X11 + improving wording