Sync data is encrypted before it leaves your device for the Sync server - that encryption uses the Sync account password. Change the password and the data can't be decrypted, so the data is erased from the Sync server as beinf useless.

See the screenshot and see the warning

Hi there, thanks on your kind response,

so I don't get it so you are telling me that because I change the encryption of firefox account to sync password all the data was wipe out?it means that all my bookmarks are gone?

Yes, your data is gone if you changed your Password for Sync. There is a warning about that (as in the screenshot), but it seems that many users either don't read it or they don't understand it,