DAHanser said

I have checked the the menu, as described in the sync instructions, and it's not on it on either of my Windows 10 computers.

Tools > Options > Firefox Account

In addition to the above :

It's the first item at the top of the Firefox menu - click the 3-bar menu => Sign in to Sync.

Perhaps I was not clear. When I said I "checked the menu" I meant what is listed after clicking on menu, that is the three bars in the upper right, which I've done now tens of times. What come up, in order, are:

my e-mail address; New Window, New Private Window, Restore Previous Screen; Zoom; Edit; Library; Add-ons; Options; Customize; Open File; Save Page As; Print; Find in This Page; More; Web Developer; Help; Exit.

I don't see "Sign in to Sync" anywhere on this list or anywhere on the submenus. Nor can I find Tools, which elsewhere I've been instructed to click on to get to My account.

I have also tried to update my version of Firefox, going to ?, About, and clicking "Restart Firefox". Firefox quits, but does not restart, and when I restart it Firefox has not updated. What is going on?

DAHanser said

Perhaps I was not clear. When I said I "checked the menu" I meant what is listed after clicking on menu, that is the three bars in the upper right, which I've done now tens of times. What come up, in order, are: my e-mail address; New Window, New Private Window, Restore Previous Screen; Zoom; Edit; Library; Add-ons; Options; Customize; Open File; Save Page As; Print; Find in This Page; More; Web Developer; Help; Exit. I don't see "Sign in to Sync" anywhere on this list or anywhere on the submenus. Nor can I find Tools, which elsewhere I've been instructed to click on to get to My account. I have also tried to update my version of Firefox, going to ?, About, and clicking "Restart Firefox". Firefox quits, but does not restart, and when I restart it Firefox has not updated. What is going on?

That e-mail address at the top of that menu indicates that you are already signed in to Sync on that device.

Click on that and the Firefox Accounts page in Options should appear.