Firefox Sync is not intended to be used as a "backup service". As noted in the article which was linked - "Important: Sync is not a data backup service. Users should continue to perform regular backups of their Firefox profiles to prevent loss of data." At best, Sync is a temporary transfer service of data between devices.
Please don't rely on Sync to save your data. Regular backups of your profile is uncomplicated and can save you hours of frustration.
See: Back up and restore information in Firefox profiles - https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
