when trying to open FireFox I get an error message that it is running and to either close or restart my PC. The Task Mgr. does not show FireFox running, so I've restarted pc twice and when I close FireFox it say that FireFox is still runing. Help

After closing FireFox and then try to reopen it I get a message that FireFox is still open and that I need to either close the program or restart my PC. FireFox does not show that it is open on the task bar or the Task Manager. I restart the PC and then open FireFox. After I close FireFox I get the same error message and have to restart the PC to open FireFox again.