Sync isn't designed to be used as a backup, so it isn't guaranteed that you can can personal data back. You can check if you have an "C:\Windows.old" folder with your previous Firefox profile folder. *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile You can copy files like these with Firefox closed to the current profile folder to recover specific data. You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:preferences</b> page. *Help -> Troubleshooting Information -> Profile Directory:<br>Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder *http://kb.mozillazine.org/Profile_folder_-_Firefox ---- *bookmarks and history: places.sqlite *bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder *cookies.sqlite for the Cookies *formhistory.sqlite for saved autocomplete Form Data *logins.json (passwords) and key4.db (58+) or key3.db (57 and older) (decryption key) for Passwords saved in the Password Manager<br>if you only have key3.db then make sure to remove an existing key4.db *cert9.db (58+) or cert8.db (57 and older) for (intermediate) certificates stored in the Certificate Manager<br>if you only have cert8.db then make sure to remove an existing cert9.db *persdict.dat for words you added to the spelling checker dictionary *permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences) *sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)