I'm afraid you may have made the mistake that many users make when it comes to Firefox Sync. Firefox Sync is not a backup service. It was never designed to be a backup service and it doesn't function as a backup service. It's designed to make sure that your data (bookmarks, password, etc) are the same between multiple devices. It doesn't necessarily have a complete or permanent copy of your data on the server.

Furthermore, if you reset your Firefox Account password before logging into your computer after the Windows reset, the Firefox Sync data on the servers gets erased. The data is encrypted using the password for your account, so when it's changed that data becomes unusable.

If you used the Reset this PC option in Windows rather than resetting by reformatting the drive, you may be in luck. Windows will sometimes move your Firefox data to the windows.old folder in the root directory of your drive.

Sometimes, you can find the Firefox profile folder in the application data in that folder.

In the future, you should make a backup of your local profile folder before resetting your computer so that your data can easily be restored.

Hope this helps.

Strange, it was not like that two years ago. I remember when I installed a fresh copy of windwos at that time, and logged into my FF account it restored everything bookmarks saved log ins, even the last session.

It's always been the case that Firefox Sync isn't a data backup service. It wasn't designed to be one.

That said, some people do occasionally have success using the service like that.