On the cloned PC's is the Profile name the same? And the same as the source of those clones? Have you tried editing the Device Name in Options > Sync?

I honestly would be surprised is Sync looks as deep into Windows to pickup an id for the device.

Is the computer name also the same on all computers? See Windows Control Panel > System

Try to disconnect from Sync and reconnect and make sure to set a different computer name.

The machines have been fully cloned, and the user name is the same. The Firefox profile is an exact copy. The machine names are now different. First time I opened Firefox, the device names on Options -> Sync were the same, I've already changed that and the three of them are now different from each other. However, Sync reports only the original one and either of them gets the tabs sent to the original one. I was reluctant of disconnecting/reconnecting, but I tried on one of the machines. Enough time has elapsed, and nope, no changes. Thanks for your responses.

Type about:sync-log in the URL bar and hit Enter.

Do you have any error logs in the two that haven't sync'd? If so, please post the contents of of one of the error reports for each device here: http://pastebin.mozilla.org/ Then provide the resultant hyperlink to your "paste".

All the macines do sync, they are just considered as a single one. In fact, as I was afraid it would happen, disconnecting and reconnecting on machine X changed the description seen by the rest of my devices (using "send tab to") E.g.: there is machine A, B, and C. B and C are clones of A. "Send tab to" reported just machine A, but bookmarks and history show pages visited also from B and C. I disconnected/reconnected from/to Sync on machine C "Send tab to" now reports machine C and not machine A (while B was never reported)

There is a machine id somewhere inside the profile and I need to change that (without reinstalling and syncing everything, of course...)

PS: I have also an Android device for the "send tab to" experiments

You can delete the signedInUser.json file in the Firefox profile folder and delete the Weave folder (Sync was formerly named Weave) to reset the Sync authentication. You can remove all sync related lines from the prefs.js file or delete the file to reset user set prefs to the default value (this is similar to resetting prefs on the about:config page). You can delete the compatibility.ini file to reinitialize the profile and rebuild caches like is done when you update.

I'll have the info in your last mail handy for future cloning. Disconnect/reconnec on cloned machine C actually forced an error to the original machine (A) and it didn't sync when started today, so I had to do the same on this one too. Thanks a lot