I use Firefox to access the internet-but when I go off the internet but don't shut down, then go back on Firefox it won't open because "it is already running". Task Manager doesn't show that it is operating so I can't shut it off. What to do?

It is in the question but to repeat. 1. When I go online I use Mozilla Firefox 2. While I am doing other work, I usually close the internet. 3. If I go back on the internet and click on Mozilla it won't come up, stating that "it is already running". 4. Despite that, when I open Task Manager, it doesn't show that it is running so I can't shut it down or open it. 5. If I log off or shut off the computer, then after I log in or reboot the computer, I can open Mozilla.

Thanks from a Mozilla fan.