Manage Account doesn't appear until Sync is setup. Each user account on the PC uses a separate Firefox Profile, which is where Sync data resides, so Sync needs to be setup in each user account separately.

https://support.mozilla.com/en-US/kb/add-a-device-to-firefox-sync

Thanks for your reply, but I'm afraid you didn't understand. The Manage Account button (and the other things that should be there, like the link to Unlink from Sync) have suddenly disappeared from the Firefoxes on ALL computers and all standard user accounts of mine, even though they were there and everything was working OK before I tried adding Sync to the one administrative account. Nothing I've tried doing has been able bring the proper dialog box(es) back.

merrillgi you must read this : https://support.mozilla.org/en-US/questions/969334 .

What do you see in the Firefox menu button drop-down list?

Is that Set Up Sync or Sync Now?

If you have "Set Up Sync" then you appear to have lost the Sync setup and you will have to redo setting up Sync.
If that isn't possible then try to delete (or rename) the weave folder in the Firefox profile folder and reset the Sync prefs on the about:config page.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

You can open the about:config page via the location bar and search for prefs that start with services.sync and reset user set (bold) prefs via the right-click context menu to the default value.

If that still fails then you can try a new profile.

Create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I found where I had written down my Recovery Key, from awhile ago when I had initially set up Sync. So I was able to to Set Up Sync again for two of my computers, as you suggested needed to be done. I have a proper Sync Options dialog box now on both computers, with Manage Account buttons on each. The two computers appear to sync properly with each other and also my Android smartphone, but before I mark this problem solved, I want to try and add my third computer and do a bit more testing. I'm going to forget about adding Syncing to my administration account, as that probably started all the trouble in the first place.

In answer to your first question, I had both a Set Up Sync... AND a Sync Now item on the menu when I first started up Firefox, but any subsequent invocation of the Tools menu showed just the Set Up Sync... menu item. Now that perhaps I have fixed things, only a Sync Now menu item is present (but why doesn't Firefox give me an indication that Sync is in progress, and then complete, like my smart phone does?)

Thank you for your help.

You can open the about:sync-progress page via the location bar to see the current status of Sync.

• about:sync-log
• about:sync-progress
• about:sync-tabs