It seems to be a feature of the latest version of Firefox. It's simply pointing out that the login page is only 'http', not 'https', so it's not as secure as it could be. The chances of anyone wanting to steal AB login data though are very slim because they couldn't do a great deal with it. (A hacker is far more likely to be looking for sensitive banking data than wanting to be able to pretend to be Petal54 or Buenchico on this site).
I haven't but know it can happen even on sites you know are safe - hopefully someone with a work round will see this even though it's not in Technology Petal.