How to fix:: "FF is already running..but not resp., first close or restart"

If after closing FF I attempt to open it again I get the message :"Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system." I have to wait a few minutes before I can access the internet again with FF. Even if "Task Manager" closes FF, it won't close. Most annoying.