You may recall that Firefox Sync changed the back end servers last year. A Recovery Key from the original Sync probably is useless now. Did you create a Firefox Account for the current Sync service? What did you reinstall -- Firefox? Windows? If you ran the Firefox uninstaller AND chose to remove personal data, or if you used something like Revo to remove Firefox, there is no undo for that. You may need to use a file undelete utility such as the following, and search for files whose names start with '''bookmarks-2016''' (bookmark backup files): https://www.piriform.com/recuva If you ran a Windows 10 refresh, you may still have your complete Firefox data intact on your hard drive, depending on which option you selected for your data. Can you check for these folders to see whether and how many old profile folders you can find? First, please turn on viewing of hidden files and folders: http://windows.microsoft.com/en-us/windows/show-hidden-files * C:\Users\''your-user-name''\AppData\Roaming\Mozilla\Firefox\Profiles ** How many profile folders do you find? If there is only one folder, it's likely your current one that doesn't have your data. * C:\Windows.old\Users\''your-user-name''\AppData\Roaming\Mozilla\Firefox\Profiles ** How many profile folders do you find? If there are folders here, one of them most likely has your latest pre-upgrade settings.