Retaining email addresses for login to Fastmail email webpage

Firefox does not retain email login addresses for Fastmail email webpage. I have used Firefox and Fastmail for many years and this has never been a problem. When I opened Fastmail previously and placed my curser on the email address box I was always presented with a choice of three or four email addresses. However now I am only presented with the first email address I entered since the Fastmail/Firefox upgrades. I have contacted Fastmail about this and they suggested it might be a Firefox issue