Sync will automatically create separate "device" ID's for each connected device / Profile. I have used Sync to "test" it on one PC, on the same user account, using separate Firefox Profiles and the same or different versions of Firefox to keep the data in those Profiles synchronized with each other. Sync works at the Profile level, not the application level. No problems at all were experienced using Sync in that manner.

Sync doesn't synchronize an entire Profile folder, only specific data files are synchronized and not necessarily in the file format that is used in the Profile for storing the data, i.e., JSON is used for synchronizing bookmarks and history, using data from the places.sqlite file in the Profile folder; other types of data may work similar. Addons aren't really synchronized, they are is installed from the Addons.Mozilla.Org and the data for those addons may or may not be synchronized; depends upon the individual addon as to whether that feature is provided for the addon automatically, if it needs to be turned on, or it if synchronizes data at all. So if you have addons that were obtained elsewhere, they won't synchronize. Overall, if you are using more than a handful of extensions that save data / user preferences, my experience has been that Sync may not keep all the data / prefs sync'd. A real crap-shoot, and before WebExtensions coming in Firefox 57 Quantum many extensions had hidden prefs to "toggle on" Sync features than weren't documented well - shame on the individual extension developers for that.

Backup and Restore is different than Sync. Doing that is like "cloning" - but problems may arise depending upon the operating system and the "path to" the Profile folder which may vary from device to device even with the exact same operating system on both. The Logon User Account username is the typical variance encountered. But when that happens it is usually due to a preference added by an extension, which is why it is nor recommended the the prefs.js file be "cloned".

IMO, preferable to using Sync when you are setting up Firefox on a new device. For existing devices with "used" Profiles, Sync will work but you could end up with many duplicates of the various types of data. Bookmarks would probably be the worst as far as duplication of data goes, as it does append any existing data; and then the existing bookmarks data will be sent to the other connected devices as "different bookmarks". Sync does not do a "bookmark by bookmark" screening for duplicates and eliminate the duplicates on any device. Some users purposely save the same bookmark in multiple different folders, as I do.

edit' --- But Sync would work fine after "cloning a Profile" to keep the data synchronized going forward.

Thanks for the detailed response! So it would be fair to assume that even with multiple "cloned" profiles on different devices Firefox would still create different Device IDs, thus keeping sync intact as expected.

(I thought I had observed the behavior that the source device had disappeared from the listed devices when moving the profile to a target device, as opposed to resulting in 2 different device/profile IDs showing up - but I would need to re-test to validate that)