The Sync server could be considered the "base". It receives, merges, and stores the Sync data that is sent to it by the 'connected' devices. It also sends data to those devices as needed to "synchronize" data to make the data the same on all 'connected' devices.

The first device that is connected 'sends' its data to the Sync server. The 2nd device that is connected to that Sync account 'sends' its data to the Sync server AND receives data from the server which is not already on the 2nd device. When completed, both devices have the same data and the Sync server has that same data, too. All 3 are synchronized with each other.

Then as new data is added to a device (let's say you saved a few new bookmarks to the 1st device), almost immediately that 'new' data is sent to the Sync server. Then when Firefox is launched on the 2nd device, Firefox looks to the Sync server within a minute or two and receives any 'new' data that isn't on that 2nd device. And should there be any new data on that 2nd device, that data would be sent to the Sync server as part of that Sync event.

But there are some exceptions to a 'full' synchronization on mobile devices, which don't 'get' the exact same browsing history 'sync' (fewer days on 'mobile' from the server) as is on the desktop / laptop devices. And The Bookmarks Toolbar folder differs between 'mobile' and 'desktop' devices. There may be other minor differences that I am not aware of.

From what I recall, Xmarks does have options that the user can set; Sync doesn't have any such options. The user creates a Firefox Account, connects to Sync, and it runs automatically and unnoticed by the user. The only 'controls' are whether Sync is 'connected' to the server (a Disconnect button and a "login" link in Options > Sync. And there's a "Sync Now" button in the Customize > Additional Tools & Features, that allows the user to 'force' a Sync event at will; it also has an 'on hover' Tooltip that shows the time of the last Sync event.

Sync is made to be 'maintenance-free' and 'fiddle-free' for the user. Nothing to do except for opening Firefox with the device is connected to the Sync account. Between the 1st 1.0 Sync version and the 1.1 Sync version a 'user control' or two was eliminated in Dec 2009 - then with the 3rd 1.5 Sync version the Password / Recovery coding & process was 'streamlined' resulting in another 'user control' being removed or made unnecessary (depending on you looked at those changes when they came in Firefox 29 in April 2014).

Ed-meister thank you very much for going to the trouble of your very clear explanation. My interpretation of that explanation therefore is that the synching function for bookmarks could simply be explained as an “accumulator”, that is to say as time goes by it looks for new bookmarks on any connected devices (at any time) and then adds them to those already there. Then it makes sure that all devices have the same “full set of” bookmarks when they are connected at some subsequent time.

In your explanation I note that there is no specific mention of what happens when bookmarks are deleted from a single device. I can only assume that a bookmark, deleted on one connected device that still exists on another device that was not connected at the time of the deletion, will be added back into the accumulation when that device is connected again. Consequently that unwanted bookmark will be added to all devices again as they are each in turn connected.

I am in no way criticizing the FF synch program. It is fine but if there was some explanation of the full functioning such as you have presented (I could not find anything else anywhere) including what happens when bookmarks are deleted then it would certainly have been very useful in my case.

“accumulator” - yes that would be an appropriate adjective for bookmarks getting added to existing Sync'd bookmarks

When a bookmark is saved or deleted on one Sync'd device, that action is almost immediately sent to the Sync server. In fact is have had two different Firefox versions (each with their own unique Profile) opened "tiled" side-by-side and deleted one bookmark - and a fraction of a section later watched that bookmark disappear from the other.

When multiple "change actions" like when re-arranging a number of bookmarks in one sitting, I have been informed by other experienced users that internally on the Firefox installation may delay sending changes to the Sync server and when the user stops "re-arranging" it then sends that 'batch' of changes to the server. Then when another device has Firefox launched it will almost immediately connect to the Sync server and it will "receive" those changes, additions or deletions, then Sync the data on that device.

I haven't tested that "batch of changes" delay myself, though. I am not a 'real' user of Sync since I retired as currently I have just one desktop PC and no other compatible devices that I can use Sync with. I only have a small number of bookmarks in my "Sync testing Profiles" and don't use those Profiles too much any longer. Basically, I just have / had been keeping up with the changes to Sync to be able to help other Firefox users by "playing" with Sync on my desktop with the 3 pre-release version 'channels' and watching the progress of development for the upcoming new Firefox Release versions for the last 4 years or so. And I quit doing that almost a year ago for a few reasons; not happy with the direction that Firefox looks to be headed.

My old netbook that I used for my former PT "retail outside service rep and detail" job for job reporting to the corporate intranet can't run a version of Firefox any newer than Firefox 3.6 due to its slow CPU / processor. And that was with the original Weave add-on for Firefox 3.5 & 3.6, the predecessor to Sync becoming a built-in feature in Firefox 4.0 . That's when I stopped being a "real user" of Sync.

Oops almost forgot to mention ... With Mobile devices - on Firefox for Android and Firefox for iOS - there are slight differences in the Sync data, especially with regards to the Bookmarks Toolbar folder on the "mobile" device; but I have no direct experience to contribute as far as what I haven't' seen happen when adding / removing bookmarks and how Sync handles those changes.

As far as more documentation goes ...

In the Contributors fora here I have been attempting to get some action towards getting a Firefox developer who has knowledge about the inner workings of Sync to write a support Knowledge Base article or articles about Sync. I started with trying to get a basic What is Sync? article re-instated; the old one that was used for the Sync 1.1 version that was used with Firefox 4.0 thru Firefox 28.0 gets re-directed and that KB article lacks the basic information explanation I think is needed. https://support.mozilla.org/en-US/forums/knowledge-base-articles/711833 No luck thus far, but that idea is getting discussed by people who work for Mozilla and has some supportive input from other volunteer support contributors, so maybe we'll get a new KB article out of that.

Overall, many of the other support contributors around here, many of whom I 'recognize' from other Firefox support fora going back to long before this support forum even existed, currently don't use Sync themselves or never did. Plus I would like to see more than "observational" or "anecdotal" information about how Sync works in a KB article for a feature as complicated as Sync has become.

Thanks for your in-put the-edmeister.

On a practical point it would seem you are confirming the intention of the synching program is to have the master set of data sitting on the server and when any device is newly connected it will mirror that data which, even as a layman, I always thought it would have to be that way. If that is the case then I have seemingly run into a technical/bug problem which has been alluded to in another thread of mine https://support.mozilla.org/en-US/questions/1107532#answer-842740 by guigs but as I did not even understand the basics of what was going on I couldn’t fathom out what this technical problem was that I seemed to have run across. I thank you for the informative link to https://support.mozilla.org/en-US/forums/knowledge-base-articles/711833 The fact that I could not find this link in the first place perhaps also helps to underline another problem that is being pointed at from different directions: a lack of a centralized explanation on the benefits, limitations and workings of FF synch. Anyway, within that link clearly there were others having issues regarding clear streamlined effortless bookmarking.

For me currently there are too many ifs and maybes about it. In contrast the Xmarks facility of a straight overriding 3–way choice when you log in of a) downloading the server data to replace local, b) uploading the local to replace server data or c) merging the two is an easy to understand controllable set of options and I will continue to use that for the time being even if it is only part way towards truly hands off synchronization. I get the impression there is an incredible glibness about synching, not just here with the FF synching application, especially in the light of what was well put by the-edmeister “a feature as complicated as Sync has become”.

Notwithstanding the fact I have announced my satisfaction with semi-automatic control of my bookmarks using Xmarks I went to see what they had to say about synching: “Install Xmarks on each browser you use, and it will seamlessly keep your bookmarks, (and optionally) passwords and open tabs in sync.” So seamless, so effortless so slinkily synchronistic but I, for one, especially when things go wrong, need step by step explanations on a kind of a time-line to exactly what is happening between server and numerous devices that may be connected at different times.

This is not for people at Mozilla/FF to respond to but my illustrative point regarding the glibness with which the subject of synchronization is mentioned on the internet in general.