Fix "firefox is already running" issue in unix

Fix "firefox is already running" issue in unix

I am trying to open Firefox in UNIX(Cent OS) but i got the following message:
"Firefox is already running but is not responding"
and Firefox doesn't open.
I tried this in command line:
kill Firefox
but it didn't work.
Also, I don't know in which directory I must execute the right commands.
How can I fix this?