While the old version of Sync will continue to work, the latest version of Firefox doesn't support adding new devices to the old version of Sync. This means that you won't be able to sync with a new device.
Mozilla will continue to host the old version of Sync for a limited time to allow for migration to Firefox Accounts. Self-hosted users will soon be able to run their own instance.
[^] # Re: Firefox Sync
Posté par Nerach . En réponse à la dépêche Firefox 32. Évalué à 2.
Le temps que «le plus gros» migre vers la nouvelle version.
https://support.mozilla.org/en-US/kb/how-to-update-to-the-new-firefox-sync#w_what-if-i-dont-want-to-update-to-the-new-sync