Did you choose one of the options for Windows 10 preserving your data (e.g., reset, refresh, or possibly similar terms)? Check for a folder named C:\Windows.old\Users\your_username\AppData\Roaming\Thunderbird If you do not immediately see AppData, make sure Windows shows hidden files and folders. To change that setting, see this article (hopefully it's similar in Windows 10): http://windows.microsoft.com/en-us/windows/show-hidden-files Do you have such a folder?