First off how many sync devices are we talking about? And which computer had the logins saved on? If this was on the one that you reformatted to another Distro and it wasn't synced to another device then chances are that data is gone. Sync is syncing between devices not to be used as backup. And how long did you leave sync on to insure it was connecting to sync options as sometimes it takes to connect if there is data to sync.?

Sync is syncing between devices not to be used as backup.

Sync connects to Sync servers and not device to device as I have mentioned to you lots of times now with proof.

ex: from https://support.mozilla.org/en-US/kb/how-firefox-securely-saves-passwords

Firefox Sync If you have Firefox Accounts and enabled the Sync functionality, your sync login data (usernames, passwords, hostnames) is fully encrypted once it's created and/or modified. However, Mozilla cannot decrypt your usernames and passwords when they are stored on the sync server.

Where is Your proof that it is only device to device as you keep claiming?