One of the ways of authentication in online services is to confirm your identity through a verification code that is sent by SMS or e-mail.
Firefox on iOS is able to automatically fill in verification codes in the input field as soon as they arrive, without having to open Messages or Mail.
It would be very handy if Firefox on macOS could also automatically access Mail and Messages, grab the verification codes from there, and paste them into the input field when authorizing.
You can read more about automatic input of verification codes on Apple's site