Merge means merge, at the time each device is set up. The data on that device is appended to the data that is already on the Sync server.

After that all the devices are synchronized with the same data (assuming all the devices have the same types of data selected for the sync process). When the user adds or removes a bookmark from device A, that change is sent to the Firefox Sync server. As each other device connects to the Sync server (as Firefox is launched on that device) the "changes" are sent to that device.

If you have other devices running at the same time, you can see the change take place, usually within seconds (or minutes, in some cases).

"Replace all other devices with this computer's data".
The data on the Sync server is replaced with the data on "this computer".

The thing that you have to keep on mind is that "sync" is done to and from the Firefox Sync server, not between the individual devices. The server is the "hub" and all your connected devices are the "spokes".

Thank you for this explanation. May I suggest at least a minimal change in wording. The first two SyncOptions explicitly mention the syncing will be with respect to the "Sync data". The third option conspicuously does not mention the "Sync data" and just plain is not accurate. I recommend a re-wording, maybe something that parallels the second option such as "Replace my Sync data with this computer's data", at least that is the understanding I now have from your reply.

May I also suggest the phrase "You can choose how your data is synchronized when you create your Sync account" might be reworded as the subtlities of whether "when you create" associates with "you can choose" or "your data is synchronized" left me less than confident of how I should interpret what the author meant. Did the author mean "You can choose when you create your account how your data will be synchronized" or does the author mean "When you create your account and when you add a device, you can choose how your data on that computer/device and the Sync data get initially integrated."

Again, thanks for the explanation.

Use Help > Submit Feedback... to send those suggestions to the Mozilla developers.

I have to agree with you, azalea4va, most emphatically.

This syncing strategy - and syncing always carries a strategy - is an amorphous conundrum, IMO. In a perfect world this hub-spoke design might appear to be useful and reliable, but we live in a world of turmoil, something programmers never seem to appreciate. And idiosyncrasies.

If a device is, or gets, out of sync, and we then try to engage syncing, the hub-spoke paradigm becomes very destructive. A device may well have a set of its own bookmarks, say, and the user may then want to subscribe and synchronise his data, there, with that on his other devices. Hub-spoke forbids it. Connection will destroy the new arrival's data. So it seems to me.

If syncing fails - and it often does: scan the forum - then, in the time before discovery, disparate sets of bookmarks may well evolve on different devices. Re-syncing, again, destroys certain data (indeed, Reset will destroy the server data). So it seems to me.

A syncing design with no capability to merge data, incorporating de-duplication, is no design of any use at all, so it seems to me.

To add insult to abuse, often a 'merge' option will actually 'append', rather than interweave old data with new. I understand we don't pay for this service (can we?), but even when I paid XMarks, it behaved in the same way as Firefox. Dear me..

Syncing has failed for me, and, frankly, I'm in a quandary as to what to do next, what to touch, for fear of a logic explosion..