Update: Please read the comments or just use killall <appname>
If you know the command
ps -A
then tou will probable also know
ps -A | grep firefox
now you can manually kill this via
kill -9 <ID>
Where <ID> is from the last output. But how to do this in one line? If your ps command knows:
ps opid= -C firefox
(should print the <ID>)
Then you can simply do
kill -9 $(ps opid= -C firefox)
Navigatone 
However, all this is moot. Use this instead:
pkill firefox
Has been around for several years now.
Good to know that my posts intented as private mnemonic 🙂 stimulate others to propose me good stuff!
Thanks a lot!
http://www.lmgtfy.com/?q=man+killall
Also you can use:
killall firefox