Note that Sync isn't designed to be used as a backup, but merely is meant to sync data between connected devices. Best is always to backup your personal data locally.

You can install the About Sync extension to check what data is stored on the Sync server via the about:sync page provided by this extension.

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

Close and restart Firefox to access the about:sync page added by this extension (see the about:about page for possible about pages).

Did you possibly reset the password of the Sync account because you forgot the password?

Resetting the password for the Firefox Account because you do not remember the password will remove all data stored on the Sync server because this password is used to encrypt your data before it is uploaded to the Sync server and only this password or a possible recovery key can decrypt this data.

Dear Sir, Thanks for your suggestion. My password was never reset, and after installing the "About Sync https://addons.mozilla.org/firefox/addon/about-sync/", I realized that there is no record of my bookmarks, while my saved passwords and history have been recovered, but the bookmarks have not recovered at all. It is unacceptable that for 3 years my bookmarks have been there but only my passwords are back.

Have you been using Sync for a long time or only shortly before installing the new Windows?

Note that syncing bookmark can take quite some time especially if you have a large amount as this goes in small chunks, so you should give it adequate time to finish uploading all data.

thanks for your reply. My bookmarks were collected for the last 3 years assuming uploading may take time, after 3 years not a single bookmark has recovered. It should be noted that I was always logged in to my account and syncing was done continuously.