もじらも数回やらかしてたけどな
Bug 233014 - default install into windows\program files directory, erases files
https://bugzilla.mozilla.org/show_bug.cgi?id=233014
Bug 233625 - Uninstalling deleted non-Firefox folders (after installing to C:\Program Files\)
https://bugzilla.mozilla.org/show_bug.cgi?id=233625