Firefox users, all day I'm getting the message "Firefox is already running but not responding" when I close it and restart. Are you?

This just started happening today. When I close Firefox and open it later, I get the message mentioned, with instructions to stop the process or reboot the system. I have to go into Task Manager and stop the process. Apparently, others have had the problem because if you Google the error message, you’ll see suggestions as to how to solve it. Is it happening to you?