Kill application
Sometimes when you exit an application (such as Firefox or Mozilla) in the normal way, it appears to shut down fully, but it's actually still running in the background. This can cause problems such as a "profile in use" error. In such cases you may need to terminate the application's running processes in another way, depending on your computer's OS.
Windows
Use the Task Manager:
- Windows XP: "Ctrl+Alt+Delete" -> "Processes".
- Windows NT/2000: "Ctrl+Alt+Delete" -> "Task Manager -> Processes".
- Windows 95/98/ME: "Ctrl+Alt+Delete".
In the list of running processes, look for "firefox.exe", "thunderbird.exe", or "mozilla.exe" (whichever applies) and terminate it if it exists.
Linux
Use the "ps" and "kill" commands. (See their respective manual pages with "man ps" and "man kill" and various other documentation online.)
Mac OS X
Go to your Apple Menu and select "Force Quit". For Mac OS X 10.3 and later, you can also use the Activity Monitor found in your "Utilities" folder ("Applications -> Utilities -> Activity Monitor").