When merely reinstalling Firefox your data wouldn't be deleted, you must have done more than a re-installation - like deleting the Profile where user data is stored.

Overall, Sync wasn't intended to be used for backup purposes; it is for synchronizing data across multiple devices, where your data would be on another device in addition to the Sync server. The difference being that it is easy to inadvertently clear data from the Sync server; where user data on each device is a lot harder to delete.

The biggest problem that we see here with regards to using Sync as you did to "backup" your Firefox data, is the user resetting the Sync Password thru the (new) re-installation when they are trying the connect to the Sync server. That action is 'fatal'. Data that is on the Sync server is stored encrypted - the encryption "key" is created from the account password; change the password and that data is rendered useless, with no way to decrypt it. So the data is deleted which allows a fresh start with Sync and also allows the user to continue using that account, eliminating the need to create a new account using the different email account when they are dealing with corrupted data 'stuck' in a Sync cycle.

So, my question is - do you think you reset your Password for your Sync account?

I did not reset the the sync password. But you are right on deleting the profile and getting a new one. So even if the sync password is not reset, by deleting the old profile, does the data on the sync server gets deleted?

No, the data on the Sync server wouldn't get deleted when deleting a Profile on a device. AFAIK, the only thing that would wipe data from the Sync server is a password reset.

Any message that a device by that name was already connected to Sync? Or something like the "device name" was being used? Or the device already exists?

I recall a recent discussion about renaming the "device name" when a message like one of those appears, but I haven't been able to check that out yet.

Any message that a device by that name was already connected to Sync? Or something like the "device name" was being used? Or the device already exists?

Nothing of the above appeared. The same name I had given before appeared after the reinstallation and login in