You receive the error “Firefox is already running, but is not responding.”
This error occurs when Firefox is closed but is still running in the background. Firefox is either in the process of closing or is frozen and hasn’t quit properly. If waiting a few seconds didn’t help, then Firefox has failed to quit properly. While all Firefox windows may be closed, Firefox itself is still running in the background. It may be frozen and using no system resources or it may be using up your available CPU. In rare situations, this may be caused by a problem with your profile, Firefox is designed to ensure that only one copy of Firefox is using a profile at a single time. To ensure this, it places special locked files in the profile directory. In rare cases, Firefox could have left behind the locked file when it crashed or quits abnormally. You can remove the lock by following this post.
First, open the Task Manager by pressing Ctrl+Shift+Escape or you can also right-click your taskbar and select Task Manager.
For Windows 8 users: You may find Firefox under the tab Background processes. You can also click the Details tab to see a full list of processes.
Type the letter f and you should jump to the firefox.exe process. You can also click the Image Name header to sort the process list alphabetically and locate firefox.exe.
Once you’ve located firefox.exe, select it and click the End Process button.
This is what the message was asking you to do when it came up with “close the existing Firefox process.”
Confirm the operation and Firefox will quit. You can now restart Firefox.
If you still get this error after following these steps, Remove Profile Lock.