From MozillaZine Knowledge Base
This article explains how to completely exit (quit) your Mozilla application.
Closing the application
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.
Start the Task Manager:
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.
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).