From MozillaZine Knowledge Base

This article explains how to completely exit (quit) your Mozilla application.

Closing the application

1. Click "File -> Exit" (Windows), "File -> Quit" (Linux) or, in Mac OS X use the "Quit" option in the application menu (e.g., "Firefox -> Quit Firefox").
2. If any remaining dialog boxes or secondary windows, remain such as the Downloads window or the Extensions window, close them. Mozilla Suite/SeaMonkey 1.x - Windows only: If Quick Launch is enabled, Mozilla's icon will appear in the Windows "System Tray" (next to the clock). Right-click on the icon and choose "Exit".
3. If you are diagnosing problems, continue with the next section to ensure the application is completely closed.

If the application fails to close completely

Sometimes the application stops responding to input, or may still be running but is not visible. This can cause problems during future startup attempts and result in error messages such as a "profile in use" or "already running but is not responding". (It is also possible you might not get an error message.)

Check for and terminate the application's running processes as described below. If you still have problems after terminating the application process, then restart the computer.

Windows

Start the Task Manager:

• Windows 7: "Ctrl+Shift+Esc" -> Processes (or right-click in a blank area of the Taskbar and select Start Task Manager, or Start -> type: taskmgr -> press "enter", select Processes).
• Windows Vista: "Ctrl+Alt+Esc" -> Processes (or right-click in a blank area of the Taskbar and select Task Manager, or Start -> type: taskmgr -> press "enter", select Processes).
• Windows XP: "Ctrl+Alt+Delete" -> Processes (or right-click in a blank area of the Taskbar and select Task Manager, or Start -> Run -> type :taskmgr -> click OK, select Processes).
• Windows NT/2000: "Ctrl+Alt+Delete" -> Task Manager -> Processes (or Start -> Run -> type: taskmgr -> click OK, select Processes).
• Windows 95/98/ME: "Ctrl+Alt+Delete"

In the list of running processes, look for "firefox.exe", "thunderbird.exe", "mozilla.exe" or "seamonkey.exe" (whichever applies) and end the process, if found. Carefully check through the entire list of processes because multiple instances may exist and end each one found.

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

Open 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).