when pkill isn't sufficient to kill firefox

Firefox hanged and I closed it. Restarting it the message came restart or stop firefox. Pkill command normally sufficient didn't suffice to kill firefox.
pgrep firefox listed two processes.
killing these processes by kill , didn't work either.

killall -9 firefox-bin

did the trick.
source: http://www.cyberciti.biz/faq/kill-process-in-linux-or-terminate-a-process-in-unix-or-linux-systems/

for the record:

This is an oldie but a goodie:

1. When a program gets hung, hit Alt + F2 to bring up the Run dialog.
2. Type in xkill and hit enter.
3. Your mouse cursor will change into a cross or skull and bones.
4. If the unresponsive application is not in focus, use Alt-Tab to switch to it. Left click anywhere on the application window to kill it.