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

UNIX:

First, check if you have firefox/thunderbird instances running

ps -ef | grep <username>

kill these with kill <pid>

Then delete the following files:

~/.mozilla/firefox/<profile-name>/lock

~/.mozilla/firefox/<profile-name>/.parentlock

Windows: Check with the task manager if firefox/thunderbird is running, if yes, kill it.

Delete the following file:

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