root@lucy-Brcko1:~# ssh -X betty@192.168.1.9
betty@192.168.1.9's password: 
Linux lucy 2.6.27-17-generic #1 SMP Fri Mar 12 03:09:00 UTC 2010 i686

The programs included with the Ubuntu system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Ubuntu comes with ABSOLUTELY NO WARRANTY, to the extent permitted by
applicable law.

To access official Ubuntu documentation, please visit:
http://help.ubuntu.com/
Last login: Wed Jun 26 19:05:22 2013 from 192.168.1.7
betty@lucy:~$ firefox
Error: no display specified
betty@lucy:~$ echo $DISPLAY

betty@lucy:~$