If you formatted your hard drive, your local Firefox data is erased and can be considered unrecoverable. Unfortunately, it sounds like you've fallen into the common issue that lots of people have with the Firefox Sync service. <u>Firefox Sync is not a backup service</u>, so it doesn't function like one. It's designed to sync your data (bookmarks, passwords, preferences, etc) between multiple devices. It doesn't usually store permanent or complete copies of your data. If you did not make a backup of your Firefox data before formatting your computer and logging into Firefox Sync did not recover your data, then sadly you are out of luck.