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Firefox 23.0 canonical 1.0 already running not shutting down or starting up error. States its already running need to shut-down do system restart and close browser.

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ok.. Open your terminal and run this command sudo pkill firefox. Then try to run firefox again.. – Saurav Kumar Aug 31 '13 at 16:27
+1, and just pkill firefox should do. – mikewhatever Aug 31 '13 at 16:35
if the two answers below which recommend killing the existing process do not work, because firefox isn't actually running, you'll want to remove the lock files as per one of the answers at… – matt freake Jul 23 '15 at 10:59

If you like the UI route:
1. Click the Ubuntu Logo
2. Type "System Monitor"
3. Find Firefox in the list of running applications, right-click and choose to kill the application

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Go to command line:

  • Run this command: $ top

  • you'll see some graphical output, then note the processID (PID) of firefox

  • then press 'k' to KILL the process
  • you'll be asked to enter the processID (PID) of process you want to kill.
  • Just enter the processID of the firefox, and you're ready to go ..!!

And you can use this procedure to kill any process.

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