Firefox 29 and later have a new Sync version that use a Firefox account and only use the name and password and no longer use a separate Sync (recovery) key (the Sync key is automatically derived from the password of the Firefox account). The new Sync version is completely different from the version that was previously used and you need to install a Firefox 29 or later version on all connected devices. You can enter the name and password of the Firefox account on each computer to connect to the new Sync account. You no longer can pair a new device by entering a pairing key on a connected device although this might be possible again in a future version of Sync if they decide to re-add this feature. You need a Firefox 28 version to connect to an old Sync account that used the Sync key. *https://support.mozilla.org/kb/how-to-update-to-the-new-firefox-sync *https://wiki.mozilla.org/Identity/Firefox-Accounts