Firefox sometimes does not recognize a username prompt

Firefox does a good job of filling in username and password prompts. Occasionally, it doesn't. The sign-in prompt for American Airlines is an example. Go to AA.COM and click on the "Log in" button. The prompt box labeled, "AAdvantage® # or username", is not filled in; but the prompt box labeled, "Password" is recognized and filled in. There must be something about the wording or the HTML used for the first box that obscures the requested information.