But Firefox already works like that. If you install as a (non-admin) user, it goes into your user dir and there is no update service.
If you install as an admin user, it goes into Program Files, and by your own admission, there's no other way to update it than to have the update service.
If you install as an admin user, it goes into Program Files, and by your own admission, there's no other way to update it than to have the update service.