Once in a while Firefox or Thunderbird does not close gracefully. This could be caused by the application crashing or simply the application didn’t do some “housekeeping” after it closed. You go to run Firefox or Thunderbird and you get the message it is already running, but it is not. The majority of the time, going into the Windows Task Manager via Control+Shift+Esc and clicking the Processes tab you will find firefox.exe or thunderbird.exe listed. Highlight the process and click ‘End Process’ and confirm. Once the process has been ended, you should be able to launch the application again.

But what if you still get the ‘already running message’ or firefox.exe or thunderbird.exe were not listed under processes? This is the case where the application didn’t do some “housekeeping” after it closed. More specifically it failed to remove the parent.lock file in your profile folder. All this file does is tells the application that this profile is currently in use and to generate the ‘already running message’ if you try to launch the application with this profile. Deleting the file and restarting the application will take care of this (and also recreates the file, but it should go away once you exit the application again). The easy part is deleting the file, the hard part is locating the file. Follow these directions to locate and remove the parent.lock file:

1. Restart Windows – this is important as likely something (Windows Updates, another application, gremlins) caused issues and was ‘holding’ on to the parent.lock file, thus not allowing Firefox or Thunderbird to remove this file upon exit. Restarting Windows will remove whatever is ‘holding’ on to the file, this allowing it to be deleted.
2. Locate your profile folder – Press WinKey+R. The Windows ‘Run’ box will appear, type the path for the application in the box and click OK:
   • Firefox: %APPDATA%\Mozilla\Firefox\Profiles\
   • Thunderbird: %APPDATA%\Thunderbird\Profiles\ 
3. Windows Explorer will open with the profile folder(s) for that application. If there is more than one folder listed, look for the folder with the most recent time stamp and double-click on the folder
4. Locate, select and delete file parent.lock
5. Close the Windows Explorer window and start the application

If you are still unable to launch the application refer to Mozilla’s Knowledge Based (while this is for Firefox, it will work for Thunderbird as well). In this case there may be a permissions issue with the profile folder or the profile folder is missing or damaged.