Originally two factor authentication was done via an SMS message with a code (not specifically by Mozilla, but in general). The reason that most companies are transitioning to using two factor authentication apps instead is for security. As it turns out, it's fairly easy to hijack SMS text messages, which would make it easy for an attacker to gain access to your account, even though it's protected with two factor authentication. Authentication apps don't suffer from this security flaw and since the majority of mobile devices are smart devices, it's usually not a big deal. Another good thing about using an authentication app instead of SMS is that you can use a variety of devices, not just those that have a SIM card. It also allows you to use multiple different devices to authenticate your account. For example, if you don't have access to your smartphone, you can unlock your account with a computer if you have an authentication app setup on there.