From what I understand, Normandy is an infrastructure for delivery of some changes to some of Firefox users (or all of them). There are two major use cases: preferences rollout and studies. In the first case default values of preferences get changed (if your pref has non-default value, it won't affect you). In case of studies some piece of code gets delivered and executed, which cat do anything. In this hotfix the study installs add-on, which in turn installs certificate.