Do you still have the Status Bar visible?

View > Status Bar

It is possible that the screen is too wide or too high and that the scroll bars fall off.
Open the system menu via Alt+Space and see if you can resize that window.
If that works then close Firefox to save that setting.

See also http://kb.mozillazine.org/Resizing_oversize_window

Thank you SO much for taking the trouble to answer, Cor-el. (And how exciting to discover there is another survivor from the planet Krypton..!)

I followed your Alt+Space suggestion, as well as the instructions from the link you thoughtfully provided, out of desperation -- because I had thought of bringing down the size of the window already.

Then I suddenly noticed your initial "View > Status Bar" idea. By God, that did the trick. So seemingly simple, and yet a solution that might as well have been on the moon for me. I am deeply grateful, and much appreciative of your kindness and of your knowledge.

Also, and although possibly not related to your problem, I have to remind you that the version of Firefox you are using at the moment has been discontinued and is no longer supported. On top of this, it has known unpatched bugs and security problems. I urge you to update to the latest version of Firefox, for maximum security, stability, performance and usability. You can get it for free, as always, at getfirefox.com.