Before diving in, I just wanted to mention that some users discover that shutting down and restarting Windows resolves post-update problems with Firefox finding its settings. If you haven't already, please try that. The Windows 10 update has various options for what to do with your old 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. Based on what you find, we can suggest the shortest set of next steps.