Sometimes a major Windows 10 update will move your Firefox data into a temporary folder named: C:\Windows.old '''Do you see a folder like that on your system?''' Firefox data is stored under the AppData path which is hidden, so in order to search for files, first set Windows to show hidden files and folders: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files Then you could explore down this path: C:\Windows.old\Users\''your-user-name''\AppData\Roaming\Mozilla\Firefox\Profiles That's where your Firefox data is stored. If you find that, we can help with steps to return your data to a more useful location. As a sneak preview, there is this article: [[Recovering important data from an old profile]].