Sync shares selected information between different systems/profiles. Any change on one is shared with all others in the sync.

You can share the whole user profile or just parts of it.

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.

Did not address the questions... It's not "How to do it... " it's "what's the end product look like and can one go back?"

This is all I have on Sync. Please look at the Wiki for visuals :

• https://www.wikihow.com/Use-Firefox-Sync
• https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer
• https://support.mozilla.org/en-US/products/firefox/sync
• https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox
• https://support.mozilla.org/en-US/kb/firefox-sync-troubleshooting-and-tips
• https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts

Once it is synced, there is no going back with data. You can un-sync devices and or disable Sync.

• https://www.bleepingcomputer.com/news/software/mozilla-will-let-users-disable-firefox-sync-starting-with-firefox-60/

There is always this instead and should do also :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Please let us know if this solved your issue or if need further assistance.

Please read the questions.

g777p said

Say I have three devices, and just with bookmarks. Does syncing consolidate the three into one hugh list or three separate lists?

It's just one list.

Is it alphabitized

I can't say. I never checked.

or attached?

What do you mean?

Are duplicates highlighted or skipped or just duplicated?

Unless the duplicates are in different folders, they are skipped

Can very old or dead link bookmarks be filtered out?

No. But there are/were add-ons that could check those links for you.

Can I return to previous if it's too messy and I don't want it?

There is no going back.

What do I backup and how to return to current status if it all goes wrong?

The Bookmarks Manager creates backups whenever there is a change. These backups only store bookmarks. Nothing else.

g777p said

Did not address the questions... It's not "How to do it... " it's "what's the end product look like and can one go back?"

NO, once the bookmarks (and other data) is merged on each device that data is changed on each device. There is no "go back" for Sync -- but the automatic bookmark backups that is in each Firefox installation saves 15 days of backups that won't get changed until they are overwritten. Those "backups" occur every day that Firefox is used, whether bookmarks change or not. If you don't add or remove, or re-arrange any bookmarks the file will be identical to the day before save.

Thus the user can "go back" with bookmarks on each device for like 2 weeks. Default pref for that is 15 (days) and can be changed by the user.

As far as the "end product" goes, I can't really say from my own experience with Sync. I have been using Sync for like 10 years, from even before Sync became a standard feature in Firefox 4.0, from when it was being developed as an add-on for Firefox 3.5 and then 3.6.

Part of the "end product" depends upon how many bookmarks are saved on each device, how you have them organized on each device (folder structure and if you are using the "tags" feature), and how you wait to make a judgement about the results, as the initial Sync on each device may change the order some in later Sync events.

Note that you can fully disable Sync in Firefox 60+ via the about:config page.

• identity.fxaccounts.enabled = false