Sync isn't intended to be used to backup Firefox data for the purpose of reinstalling the operating system. One copy of Firefox data that was merged from all connected devices is saved on the Sync server for your Firefox Account; no backup copies are saved. https://support.mozilla.org/en-US/kb/firefox-sync-troubleshooting-and-tips The most common problem that user's who attempt to use it as a backup medium is with resetting the Firefox Account Password when they have a problem reconnecting to Sync. Evidently they failed to take heed of the warning. ''see the screenshot'' Changing the password without having their Firefox data on another device is a fatal error as the Firefox data is wiped from the Sync server, as stated in that Reset Password dialog box for about:accounts. The original password was used to create the dual kA & kB encryption / decryption keys; a different Password won't decrypt the Sync data making the existing Sync data useless. ''Resetting the password has another function; to clear corrupt data from the Sync processes & server without the user needing a different email address to create a new Firefox Account. Personally, I went thru 7 email addresses with the earlier Sync 1.0 & Sync 1.1 versions; the current Sync 1.5 version (as of Firefox 29) fixed that and gives the user the opportunity to start a fresh with Sync by resetting the account password.'' https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information