It's a complete non-starter on a computer with multiple user accounts. How do you update it? Do you really want to update every single version separately? Really? What about corporate environments?
Firefox installer isn't great for corporate Windows environments either because it isn't delivered as an MSI package. Why on earth the FF people can't follow a nearly 10 year old platform packaging standard is beyond me. Yes you can get FF MSI packages from 3rd parties but that has its own problems and barriers.