Firefox does not close when I leave the Internet

I have recently been having a problem with Firefox.

When I close down the Internet, and then go back and click on the Firefox icon to restart the browser, a screen comes up that says:

"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."

[Image of this message below.]

Since I don't know how to "close the existing Firefox process," I am having to restart the computer.

Can you please help me resolve this?

Thanks so much,

cashcqshel

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