See this KB article - [https://support.mozilla.org/en-US/kb/how-to-update-to-the-new-firefox-sync?esab=a&s=upgrade+Sync&r=2&as=s How to update to the new Firefox Sync]

*Firefox 29.0 and later use the 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). *This version is completely different from the version previously and you need to install Firefox 29.0 or later versions on all connected devices. *You can enter the name and password of the Mozilla account on other devices that have a Firefox 29.0 or later version to add extra devices. *The new Sync version used in Firefox 29 and later will still use a recovery key to encrypt data locally before uploading, but this recovery key is generated internally from the password of the Firefox account, so you no longer need to worry about it. * https://wiki.mozilla.org/Identity/Firefox-Accounts