Warning! Using sync, and knowing the data on each member of the sync is different, may overwrite the wrong data. In other words, in your example, B could overwrite A's data.

It would be best to try this first: On computer B, create a new blank profile. Once created, have that profile sync. Later, any data in the old profile that you want you can Copy to the new profile. But be careful.

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address box.

You can use sync to share passwords, bookmarks, and history. Anything else may not be compatible between different browser versions or computer systems.

https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer Sync your bookmarks, history, passwords, add-ons and open tabs with another copy of Firefox. Learn how to set up Sync.

https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox We'll show you how to choose which types of information (bookmarks, history, add-ons, and passwords) to share across all your devices through Firefox Sync.

https://support.mozilla.org/en-US/kb/firefox-sync-troubleshooting-and-tips

https://support.mozilla.org/en-US/kb/sync-firefox-bookmarks-and-browsing-history-iOS Sync your passwords, history, tabs and other browsing information across your iOS, Android and desktop computers with Firefox Accounts.

I do NOT understand how Mozilla can be so damn reckless with our setups! I have this exact same problem and frankly am ready to bail on the otherwise good idea of sync. How can I tell which setup will actually be used? I have a setup with a decade or more of passwords, over a thousand open tabs, and thousands of bookmarks as well.

I cannot chance losing all of this and for the life of me I cannot understand why nobody at Mozilla sees this as a problem. Even the respondants here only know enough to tell to you be careful, or not do it.

Which machine's settings are stored for syncing? Anyone know? Is it the first machine you sync? And why is that not explained or able to be controlled by the user? It's almost like someone created sync and never bothered to use it themselves. The guidance is poor poor poor!

Someone with experience with this PLEASE weigh in here.

Okay, I'm wading thru all the different articles and options of "sync" which really should be called "merge" but anyways with my machine I want to use as my primary location, instead of sweating all this "sync" craziness I can right click a tab, choose "select all tabs", and then choose send tabs to the device I want to them to be duplicated to. (Addendum .. this only copied over 100 tabs for some reason so I guess there is no perfect answer)The only downside is that it appears that pinned tabs are not included for some reason when you choose to select all tabs even though they are highlighted. Maybe those just take longer?

Anyway, it sure would be nice if I could also do that with bookmarks, settings, etc too, and just not use sync at all.

That process makes about a thousand times more sense than sync. I just wish I could do that with all the data types that sync provides like bookmarks. It's logical, easy to understand what Firefox will do and gives the USER some control over a process that's otherwise about as opaque as you can make it.

I hope this helps other users with the same issue in some way.

You would have to clear all data on that device that you do not want to Sync to other devices before connecting that device to Sync,

Well, it's far too late for that. What a hacked up mess this thing is!