Windows 10 has a setting that hinders or permits from apps to run on the o.s.. This is a feature to protect the o.s. from unauthorized installations of apps. So it could be that the permission for the current windows user has been limited to using apps but not installing them. If you go to the windows control panel/management console for users, you may find that the setting for permissions needs tweeking. In addition to the above, you can also try rebooting windows. Sometimes the ol'file permissions have locked up the files. So rebooting the os can release the file lock(s) and set your FF on the right course. Let us know what you find ! :-)