Just the messenger...

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

Sync is used to sync your browser data between devices. So when you logged into your Firefox account with no data, that is what synced.

I learned the hard way awhile ago about how sync works which is why I have several devices synced. see screenshot

I think what happened was when you connected to sync, the servers thought the data had been removed from your profile. Thus it removed that data from the servers.

Back up and restore information in Firefox profiles {web link} I use a Legacy add-on that makes backups of my user profile, Just In Case. Unfortunately, it is not compatible with the current Firefox.

Every now and then, open the profile folder and create a copy of the current profile as is to another location on your hard drive. This way you will have a backup.

If you do this each day, you will not lose much data. This should be done with Firefox Closed.

Note: In case you need to restore from these backups, you may need to overwrite old or corrupted files.

You can install the About Sync extension in Firefox 72 and newer on desktop to check what data is stored on the Sync server via the about:sync page provided by this extension.

• About Sync https://addons.mozilla.org/firefox/addon/about-sync/

Close and restart Firefox to access the about:sync page added by this extension (see the about:about page for possible about pages).