HOW TO FIX: Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

This is a quick and easy solution for the error on Firefox that says “Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.” This will appear even though you restarted your computer and even if you uninstall and reinstall Firefox.

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In order to make Firefox work again, you will need to delete the lock files that tell Firefox not to start another process for the current user. These files are usually removed by Firefox, unless it shuts down abnormally or suddenly.

First, we will need to locate the lock files – depending on your operating system, they will be in a different place:

Operating system Profile folder location(s)
Windows NT (NT 4.x, 2000, XP, and Vista) “%APPDATA%\Mozilla\”
Windows 95 (without Desktop Update) C:\Windows\Mozilla
Windows 95 (with Desktop Update)/98/Me C:\Windows\Application Data\Mozilla\
C:\Windows\Profiles\\Application Data\Mozilla\
Unix/Linux ~/.mozilla/
Mac OS X ~/Library/Mozilla/
~/Library/Application Support/

If you’re running Windows XP or Windows Vista, you can find your %APPDATA% folder by first pressing [Windows Key]+[R] → Then, type in %APPDATA% → click OK.

A Windows Explorer window will open, showing the contents of the C:\Documents and Settings\\Application Data folder in Windows 2000 and XP or the C:\Users\\AppData\Roaming folder in Windows Vista. You can then open successive folders until you get to the profile folder.

Look for the corresponding lock-file for your system and try to delete it:

The application may have shut down abnormally, leaving the lock in place. To fix this, open the profile folder and delete the file,

* “parent.lock” (Windows),
* “lock” and “.parentlock” (Linux), or
* “.parentlock” (Mac OS X)