Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

Note that Sync isn't meant to be used as a backup, so relying on Sync to restore your personal data after reinstalling Firefox and possibly removing the Firefox profile folder. Normally reinstalling Firefox shouldn't affect your personal data, but if you also remove the profile folder without first making a backup beforehand isn't a good idea. If you have reset the password of the Sync account that this causes data on the Sync server to get removed.