Sorry, but if you would have done an "unlink" on each of those devices to disconnect them from the Sync service, before those devices became unavailable, they wouldn't appear in Sync any longer.

The one time I did that, I created a new Sync account to get rid of the "ghost" device. Changing the password might be a better solution than what I did.

Thanks for the reply, though I hoped that it was possible to unlink the machines. Seems this problem is unsolvable at the moment.

This needs to be addressed, so older OS versions links can be removed where necessary.

Is hard to "unlink" devices which no longer work.

Might sets of stored data from devices be removed after a period not used ?

You can log in to the Firefox account to connect to Sync and log out to disconnect. This won't remove data stored on the server.

Data stored on the Sync server from unused devices will be removed AFAIK, but this can take quite some time (longer than a week), so you will have to wait for that to happen. If you want to do this immediately then you will have to change the password of the account as that affects the Sync key and thus resets all stored data. In previous versions you would still have to generate a new Sync key to have this effective immediately.