Just don't set it up. There is no on/off switch now and from what I have read I don't think they're planning on having one, either in the UI or hidden in about:config .

There is pref named services.sync.autoconnect in about:config that is set to true by default that you can toggle to false so the sync service won't automatically try to connect to the server, but sync shouldn't be trying to connect anyways until the user sets it up.

You can disable sync. Just go to Options - Sync and click "deactivate this device" at bottom.

Good point you make, but that option doesn't even appear unless Firefox Sync has already been setup by the user in that Firefox Profile.

My sync got activated some how , but not by me, this info helped me turn it off, thanks !