Suppose Firefox doesn't react. I do
ps -A
and get
Code:
...
 5821 ?        00:00:00 ruby
 6600 ?        00:00:00 gconfd-2
 6610 ?        00:00:00 mozilla-launche
 6619 ?        00:02:51 firefox-bin
 7330 ?        00:00:39 java
 7368 ?        00:00:01 konsole
...
This tells me that firefox-bin, the actual Firefox executable, has PID 6619. I can now send it a SIGTERM, asking it politely to quit right now, kthxbai.
kill 6619

If Firefox doesn't react because, for example, it hangs in deadlock and even signal handlers don't get through, I can send a SIGKILL, telling the OS to just kill the stupid thing.
kill -KILL 6619