I have a strange problem with login form o Firefox (tested version 49.0.1 on Windows and 48.0 on Ubuntu). When I click the Visa Checkout button, login form appears. The password field should be empty (it has autocomplete="new-password" attribute). But Firefox ignores that and autocompletes that field.
When I click "Sign In to Visa Checkout" I'm getting an error "Please enter your sign-in credentials". But when I click the button one more time everything is ok.
Maybe you shuld turn off checking whether the field has been clicked? Or maybe is there another way to turn off autocompleting for Firefox?