The Windows 10 installer may default to moving your application data (including Firefox data) to a 30-day backup folder (C:\Windows.old) but there is a way to tell the installer to leave everything in place. That would be simplest. Even if you need to reinstall Firefox, as long as its data folder wasn't moved, the Firefox installer will be able to discover your old data and get you back to where you were. Note that you might need to reinstall some plugins like Flash and Silverlight, since those are stored outside of Firefox's data folders. Let us know if you discover anything important missing. Finally, it's never a bad idea to create a backup of your system before a major change, or in case you need to undo it. This article has info on backing up your Firefox data: [[Back up and restore information in Firefox profiles]].