I should add that I could simply restart my computer but it would be nice if the browser (a great one!) would close when I tell it to.

Do you have the Firefox icon on your quick launch? Right click on it, and select Open New Window. Sometimes this works. If it opens, close it by Menu > File > Exit.

This is not a cure but will make it easier if Firefox locks up in Windows. https://support.mozilla.org/questions/997866 {web link}

Thank you. I tried that and still have the problem. I usually right clk the icon to close. Sometimes ccleaner says ff is still open, come to think of it and offers to close it. I will just do that. Thanks.