Sorry to hear about this problem. I'm not sure why Sync didn't bring over your data.

You can check what data is in the Sync cloud for your Mozilla Account using the About Sync add-on:

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

After you install the add-on, you can type or paste about:sync in the address bar and press Enter to load it. The page shows each category of data currently found in the Sync cloud.

Anything on the server in the Bookmarks category?

If not, Windows might have created a temporary backup folder during your reinstall. This assumes you didn't start the process by erasing or reformatting your disk. Can you find this (using File Explorer > My Computer This PC > C):

C:\Windows.old

If you do see that folder:

Set Windows to show hidden files and folders -- this Microsoft article has the steps:

https://support.microsoft.com/en-us/windows/view-hidden-files-and-folders-in-windows-97fbc472-c603-9d90-91d0-1166d1d9f4b5

Then explore that folder up to this point (if it exists):

C:\Windows.old\Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles\

If you find recent data there, please pause and we can suggest how to restore old bookmark backups.

Did you reset the password for the Sync account because you forgot this password?

If you reset the password for the Mozilla account because you do not remember this password, then that will remove all data stored on the Sync server because this password is used to encrypt your data locally before it is uploaded to the Sync server and only this password or a possible recovery key you created earlier can decrypt this data.