The support for the non-standard Web Payments API, the
navigator.mozPay method in particular, has been removed with Firefox 51. This API, drafted by Mozilla to enable in-app payments on Web applications, was initially implemented in Firefox OS and later enabled in Firefox for Android and the Web App Runtime of Firefox for desktop, but it has never been widely used. The Mozilla Marketplace is removing the payment support, while Firefox 47 has already removed the Web App Runtime from desktop and Android.
In the future, Firefox may implement the standard Web Payments API which is being designed in the W3C Web Payments Working Group.