With the current versions, try this;

Right click on the 'hamburger' icon at the far right on the toolbar. Click on the first line which should be the sync option.

Select Manage Account.

Find Device Name and enter a name for that unit. Do the same for any other units in the sync.

As far as fixing that issue on the "cloned" Profile, I concur with Fred so far as changing the Device Name - - but only using about:preferences#sync > "Changed Device Name" to see if that fixes it. That worked for me on a Win7 computer a 4 or 5 years ago, after I 'cloned' a Profile which was using Sync and I wanted 'copy' of that Profile so I could use Sync to synchronize 2 Profiles on the same device in different Logon User Accounts. Part of the process to start using a new computer; a process that failed as far as extensions losing all their settings and data - as explained further along in my posting.

Sync doesn't go by the unique 'hashed' Profile identification as used in the Profile Manager, and as seen in the Profiles folder and the profiles.ini file. Sync on the browser connects to the Sync server thru the Profile which is running; the prefs don't 'go thru' the Profile Manager or use the "Profiles" part of Firefox. The Profile Manager system has been around since Firefox 0.9 in 2004; whereas Sync came about with Firefox 4.0 in 2010.

IMO, this KB article has a few errors or faults - https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles#w_restoring-a-profile-backup -

In about:config this preference services.sync.client.syncID;jtFSgWJyJeE7 is used to identify each Profile with the Sync System, and maybe with the Firefox Accounts (FxA) system too.

That said, services.sync.client.name;Firefox using Quantum is the 'label' used in the Sync Device Name field in Sync > Preferences -> Device Name.

IMO, what's happening is that copying the prefs.js file put you in the situation where two Firefox Profiles are labeled as the same Profile / installation to the Sync server, and maybe with the Firefox Accounts (FxA) log on system.

In about:config there is a column labeled Status with a value of either default or modified. The modified prefs get written to the prefs.js file. Actually it's a 2-way exchange; if you edit prefs.js file the edit will appear in about:config after a Firefox launch. And prefs.js editing needs Firefox to not be running. So basically, the prefs.js file contains the preferences that are not the "default" prefs from the original installation of Firefox.

The prefs.js file contains (only) preferences that have changed from the 'default' values, either manually thru the about:config panel by the user, or thru the Preferences / Options UI. Also, installation of extensions may add about:config prefs

IMO, the prefs.js file should not be "cloned" or you may run into a variety of issues. There has been at least one other KB article that states that prefs.js shouldn't be copied over when transferring Profile data to a new or different Profile; that includes 'cloning' a Profile.

Not so bad in Mac OSX or Linux OS's, but on Windows that can be a real 'can of worms'; although with prefs for extensions the new Quantum versions have greatly lessened the complications by providing alternative storage files for extensions prefs -- available for extension developers to use as an alternative to the prefs.js file.

(wow, the "post a reply button" was well hidden in the notification email!)

I may not have made it very clear, but I did change the device name via `about:preferences?entrypoint=fxa_app_menu#sync`. What I didn't mention is that I'm using the Developer Edition (so I can use unsigned extensions on Mac, Linux & MSWin, including some which were obtained with the "Chrome Store Foxified" extension).

I've also tried to disconnect one of the duplicate "devices" via the online device manager (`https://accounts.firefox.com/settings/clients?service=sync&context=fx_desktop_v3&entrypoint=fxa_app_menu&uid=UID&email=XXX@YYY`), maybe I should disconnect both? Presuming the reconnection is automatic when you force a sync from the disconnected device?

Other than that, is there anything else I can try? `prefs.js` is a JavaScript and thus text file, so if "something in there" is the culprit it might be possible to change it?

FWIW, I did *not* lose my extension setting, as far as I can tell, but I *may* have copied the contents of the cache directory too.

(I have never used or enabled the email notification for this forum. so I don't know what that looks like. Over the years with the number of postings I have made here, my inbox would have been filled every day. Plus I can follow the threads that I have answered right in this forum software.)

As far as editing the prefs.js file goes, yes with Firefox closed using a simple text editor like Notepad it is easy to remove any lines with "hard file paths". Just make sure you don't leave any line gaps or string two lines of prefs together. Then with Firefox running go into about:config and "set" the preference value there. But you have to make sure the edited file saves as a prefs.js file and not as a prefs.js.txt file as some users have had happen when using a text editor. If that happens Firefox will just ignore that file and create a new prefs.js as part of the launch routine.

I haven't fallen for the "let's add a .txt extension" for ages but thanks for the heads-up anyway :P

A quick look in the prefs.js file turns up this:

user_pref("services.sync.client.GUID", "XXXXXXXXXXXX"); user_pref("services.sync.client.name", "Someone's FFd on Some Machine"); user_pref("services.sync.client.syncID", "YYYYYYYYYYYY"); user_pref("services.sync.clients.devices.desktop", 2); user_pref("services.sync.clients.devices.mobile", 0); user_pref("services.sync.clients.lastModifiedOnProcessCommands", 1530954287); user_pref("services.sync.clients.lastRecordUpload", 1571558598); user_pref("services.sync.clients.lastSync", "1571577821.98"); user_pref("services.sync.clients.lastSyncLocal", "1526721825684"); user_pref("services.sync.clients.syncID", "ZZZZZZZZZZZZ");

I guess I'll have to tinker with changing one of the 1st two of the ID variables, or even the last one - undoubtedly each time disconnecting the device from the account management page first. It looks like one should be able to do this from the `about:config` page.

OK, so for some reason those services.sync keys are not present in the prefs.js on the MSWin computer (and the sync settings page looks different).

Fortunately, I finally found a helpful answer by searching for the *.GUID key

In short: after cloning the profile you need to remove the `signedInUser.json` file from the cloned profile directory. It may help to change the device name in `prefs.js` before starting FireFox the first time but I don't think that's necessary if it only serves to help the user recognise which device is which.

Curiously I had to jump through account verification steps on the original FF install (on Linux) after I disconnected it - but I did not have to restore my settings as to what's to be synced. I did not have to do the verification steps with the cloned and now independent profile on the MSWin computer BUT I did have to untick the "History" and "Passwords" sync options (I don't want those to leave my LAN).

That was very good work. Well done.