I had a bit of an issue with a Win10 upgrade from Win7 for my wife’s laptop. Nothing short of a full reload of Win10 would stop FileExplorer from resetting and restarting every few seconds. This did not allow me to use Control Panel, either. Settings worked.
So, it told me that it removed Thunderbird and Firefox.
It ALSO removed all the AppData files (like all of her email).
Fortunately, I had a full backup for most things and the stuff that wasn’t on the backup downloaded from the mail server as we keep mail on the server for 30 days.
But it was quite a scare.
Not nice, Microsoft.
Be warned and be safe. Machine working fine now.