I was actually curious about this as well, since it's been a while since I signed up for Firefox Sync. I can't say this is the first time it's been asked here.

I created an account to test with two different computers and two different sets of bookmarks. They were merged onto both systems. So the bookmarks from #1 were added to #2 as well and the bookmarks from #2 were added to #1. Then if you add a new device #3, it gets all of the bookmarks.

That's how I expected it to happen, since Firefox Sync's purpose is to mirror changes to all of your devices.

Hope this answers your question.

I'm not sure about that. What happens if you remove a book mark or a saved login from one. Does it also disappear on the others. It shouldn't with your logic.

On device #1 I deleted a bookmark and added one and then synced. I now have two duplicate sets of bookmarks, except that the first set is missing the bookmark I deleted; and second set is missing the one I added. So it appears, at least in this case that rather merge the book marks, it appends those from the other device. What would happen if I had 3 devices?

I then deleted a saved login on #1 and synced. No change.

I then went to device #2 and synced. Again, I have two duplicate sets of bookmarks, one missing the bookmark I deleted and one set is missing the one I added. In this case it simply copied the bookmarks from #1. Incidentally, the save logon I deleted from #1 is also deleted on #2.

A rhetorical question. Is there a reason why there appears to be no documentation for how firefox sync actually works. After all we are dealing with computer logic. Trying to induce the logic by experimentation gets old fast.

If you ask me, syncing should involve a master file in the sky where you can upload changes to the master download the latest master state.

Sync is short for synchronize - "make the same". The Sync server accepts the data which is sent from a device; the server holds that data and when another device is connected the server accepts that data; the server then combines the collected data; the server then sends the data to each device as it re-connects to the server based upon which data is missing from that device.

Paul said

On device #1 I deleted a bookmark and added one and then synced. I now have two duplicate sets of bookmarks, except that the first set is missing the bookmark I deleted; and second set is missing the one I added. So it appears, at least in this case that rather merge the book marks, it appends those from the other device. ...

Sounds like your bookmarks are corrupted. I wonder if one of those services is running Firefox for iOS or Firefox for Android? Dozens of support threads around here about "duplicated bookmarks / folders" issues connected to Mobile Firefox & Sync.

On Desktop / Laptop Firefox open Help > Troubleshooting Information and way down that display at Places Database use the Verify Integrity button. That should fix the issue of duplicated bookmarks and bookmark folders on that particular device; use Verify on other affected devices as needed. Mobile Firefox (iOS & Android) don't offer Verify, from what I have heard; I don't have a smartphone with Firefox Mobile.

When playing with Sync for testing deletions it's probably best to close Firefox and restart it to get a fresh opening connection back to the Sync server. Some changes - deletions / additions - get sent to the Sync server immediately but if you are doing multiple deletions / additions Sync seems (to me) to switch to a "batch mode" where the changes are saved and then sent to the server as a "batch". Although that may have changed recently, I haven't tested those actions in the last few years.

As far as "I have two devices and device #1 is the way I want both." The user has no control of how or which way data gets 'synced'; data is merged on the Sync server and all devices carry equal weight, there is no "primary device" with Firefox Sync. But on Mobile iOS / Android bookmarks saved on Mobile are treated a bit differently on the server and will appear in a "Mobile" folder on Laptop / Desktop Firefox installations.

Paul said

I'm not sure about that. What happens if you remove a book mark or a saved login from one. Does it also disappear on the others. It shouldn't with your logic.

Based on my experience, having used Firefox Sync for the past 5 years at least, Firefox Sync should remove a bookmark from all systems if you remove it from one. That's the way that it's worked for me.

OP here. Thanks for all the advice. When get time (a lot), I'll try to digest it.

You can look at this comment:

• https://dxr.mozilla.org/mozilla-release/source/services/sync/modules/engines/bookmarks.js#311