Sorry, but that is not how Sync works.

https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer#w_connect-additional-devices-to-sync

Important: Sync is not a data backup service. Users should continue to perform regular backups of their Firefox profiles to prevent loss of data.

It allows users to partially synchronize bookmarks, browsing history, preferences, passwords, filled forms, add-ons, and tabs across multiple computers and devices.

This is why I run multiple versions of Firefox. Sync will see the versions as different devices and you can backup or "Sync" between them. About 2 weeks ago there was a Nightly(on openSUSE)update and when it came back up, all my 40+ tabs in 2 windows were gone. Because I run 3 other versions of Firefox, they all had the tabs from Nightly, so I was able to just send all the tabs to Nightly. I also bookmark my open tabs.