Sync just keeps track of the installed extensions. It will install extensions fresh from the AddonsMozilla.Org server (AMO), it doesn't actually "synchronize" the user's installed extensions from their devices. And if a WebExtension isn't available from AMO, it won't get "sync'd" at all.

As far as extension prefs and data, used to be hit or miss as to what would get sync'd. IMO, too much variance in how extension developers would save prefs and data over the time that Legacy extensions were around (~ 15 years). IMO, documentation of hidden prefs for individual extensions to "turn on" Sync for prefs and data was very poor to non-existent in too many cases. I guess the user was expected to be "fluent" at reading obscure preference names in about:config and automatically "connect" those names with each of their extensions to be able to use some extensions and to get those extensions to work to their fullest with Sync.

Hopefully the situation will be better with Quantum, with the "rules" and what can & can't be done with WebExtensions is a lot narrower than in the past with Legacy extensions.

OK, thank you very much for your clear and detailed response and explanation!

There is still one point on which I am still unsure and remains unclear to me... Will performing a "Sync" ever remove any data, extensions or settings (from a customized setting back to default setting) from a device when it is sync'd for the first time, or the original device after a new one is added to the sync account? If so, would you please provide some more details about that? Such as, what types (if not any/all) of data or settings would be removed or reset to defaults?

Thank you so much!

With the initial Sync event for each device those device's data will get merged on the Sync server so each device has basically the same data. But there are some exceptions, like with "mobile" devices all bookmarks from a "desktop/laptop" Firefox won't get merged, and browsing history to "mobile" is less, too. And addons from "desktop" won't get sync'd to "mobile" and vice versa. But the other types of data should go to "desktop" from "mobile" - I think. I have no mobile devices to try it with, I use a simple "feature" phone that is 12 years old.

I avoid family parties because the nieces and nephews spend their time with their smart phones rather than communicating verbally with their guests instead; I am the family old curmudgeon! IMO, Smart Phones are dumbing-down society and affecting interpersonal communication negatively. Also prevents too many people from having a "real life"; they live vicariously via "dumb down" devices, almost like broadcast TV was accused of back in the 1950's.

And the community where I live on local cop has Tuesday "ticket the texters" and those holding cell phones and not using Bluetooth; ticket for people who have Bluetooth built into their vehicle and a lecture / warning for others not using an earpiece. wired or Bluetooth. Local law AND state statute, too; local saves "points: with the state and the fines pay more to the village, but are cheaper.

The concept of Sync is to keep all data equalized over time; delete one data entry on one device and Sync removes it on the other devices - assuming that data type is still selected to be sync'd on each device. Won't be deleted on any if the device you delete data on isn't selected to be active with Sync any longer; same for other devices - data deleted on other devices won't be deleted on the "current" device if that "current" device has that type of data not selected when that data is deleted on another. Data types can be turned off on each device without disconnecting from Sync, which allows Sync to keep working for that data type on other connected devices.

So if you setup Sync with specific data types selected and don't change the data selections everything will continue to be sync'd as with the initial "setup" Sync event.

As far as a Refresh or Reset of the Profile, (if that is what you mean by "setting back to default setting" - I recommend Disconnecting from Sync on that device, otherwise Sync maybe liable to think you want to all your devices to be reset to default. And maybe let a few days go by before reconnecting that device to Sync. But I have no direct experience with Sync and a Refresh of a Firefox Profile, as I have no real need for Sync and just "play with it" to stay current with what changes have been made over time; and have lost interest with even doing that "playing" with Sync for awhile now. Getting tired of messing up my Firefox installations and Profiles "testing the limits" and feature changes with Firefox and its features.

I am no "fan" of the Refresh feature as the first time I tried it I had a horrible experience. I was prepared for Firefox to be screwed up, but somehow the Refresh procedure forced an OS restart and a known Bug in Windows 7 came into play; my entire operating system got "flip-flopped" - User accounts got flipped, the Admin to the Limited user and vice-versa. I blame it on the convoluted manner I launch Firefox Profiles which probably triggered the OS restart. That "convoluted" manner with Firefox Profiles worked fine for me for like 12 years (and many other advanced users, too) and ever since that event and my re-installing "everything"; but sure as hell ain't gonna mess the "Refresh" again. Didn't affect any data for anything, as that is all on a different hard drive. Four separate identical 250 GB drives for 1TB of hard drive space; 2 for different OS's, one for data and one for backups, with a slice of that used for browser caches (defrag runs faster on a small logical drive, and cuts down on fragmentation of the OS and data drives, they those drives don't need defrag very often with Windows on the OS drive.)

And as far as addons Sync goes, install an extension type addon on one 'desktop/laptop" and it should get installed on the other "desktop/laptop" devices; I don't think that holds true for "mobile" devices as the extensions for each type are vastly different from "desktop/laptop" even when the "name" is the same - the internal names, the GUID's differ (I believe) between the devices.

Sorry for the rambling, just in that sort of mood this late. Hope that helped you with your questions an Sync.