Why is Firefox/ Thunderbird telling me "already running", "profile in use" or "No running window found"?

Linux

1. Change into your Mozilla profile directory:

   cd ~/.mozilla/firefox/<profile-name>.default/
   
   # For Example ~/.mozilla/firefox/torktynj.default/

2. Check the lock file.

   ls -l lock
   lrwxrwxrwx 1 username stud 19 2013-03-20 16:02 lock -> 129.132.3.132:+6060
   
   # In this example the firefox instance is running on
   # 129.132.3.6 which is tardis-a01.ee.ethz.ch

3. Login to the given machine and kill any firefox process:

   ssh 129.132.3.6
   ps -ef | grep firefox
   kill -9 <pid>
   
   # Example:
   # ps -ef | grep firefox
   # username  6060     1  0 Mar20 ?        00:01:23 firefox
   # kill -9 6060

4. Remove any non deleted lockfile.

   rm ~/.mozilla/firefox/<profile-name>.default/lock
   rm ~/.mozilla/firefox/<profile-name>.default/.parentlock

Windows

1. Check with the task manager if firefox/thunderbird is running, if yes, kill it.
2. Delete the following file:

   W:\Application Data\Mozilla\Firefox\Profiles\<profile-name>\parent.lock

Mac OS X

1. Check with the Activity Monitor if you have any dead firefox processes still lingering around. If so, kill them (Button labeled "Quit Process").
2. Delete the following file:

   ~/Library/Application Support/Firefox/Profiles/<profile-name>/.parentlock

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