yliu@bkli03l007:~$ xkill
Select the window whose client you wish to kill with button 1....
xkill:  killing creator of resource 0x3c00df1
yliu@bkli03l007:~$ firefox &
[1] 1338
yliu@bkli03l007:~$
(firefox:1338): GLib-GObject-CRITICAL **: g_object_ref: assertion 'object->ref_count > 0' failed

(firefox:1338): GLib-GObject-CRITICAL **: g_object_unref: assertion 'object->ref_count > 0' failed

(firefox:1338): GLib-GObject-CRITICAL **: g_object_ref: assertion 'object->ref_count > 0' failed

(firefox:1338): GLib-GObject-CRITICAL **: g_object_unref: assertion 'object->ref_count > 0' failed
yliu@bkli03l007:~$ ps -e | grep fire
1338 pts/31   00:00:07 firefox
yliu@bkli03l007:~$ kill -9 1338
yliu@bkli03l007:~$ firefox &
[2] 1478
[1]   Killed                  firefox