Firefox Sync is not a backup service. It's designed to transfer data from one device to another, but the storage on the Firefox Sync servers is both temporary and fragile and is not stable enough to be (and is not designed as) a reliable backup service. Additionally, because your data is encrypted, some actions (like changing your account password) will cause data to be erased on the Firefox Sync servers. If you have Firefox running on another device, you may be able to recover your data using that device. See the [[Recover lost or missing Bookmarks]] article. Unfortunately, if you don't have the data saved somewhere else, no one on this forum will be able to assist you.