kill processes by name

`killall -9 firefox`

Sometime process exists and cannot be killed due to:
- being zombie. I.e. process which parent did not read the exit status. Such process does not consume any resources except PID entry. In `top` it is signaled Z
- erroneous uninterruptible sleep. It should not happen but with a combination of buggy kernel code and/or buggy hardware it sometime does. The only method is to reboot or wait. In `top` it is signaled by D.