Did you try one of these:

• orange Firefox button > Exit
• File menu > Exit

If Firefox does not shut down cleanly, it could be hanging during its clean-up process. This article has some suggestions for that: Firefox hangs or is not responding - How to fix.

I typically have 2-3 dozen Firefox windows open at any given time, with 50-100 tabs open between them. I thought I had already tried the Exit command and it only shut down one window at a time. But, at your suggestion, I tried it again and it shut down all the windows (27 at that particular moment) at once in a matter of seconds and Firefox opened them all up again just as fast. Thank you for your help.