I have experimented and found that having multiple account names / options for autofill (two in this case) was causing the issue. However this has not been an issue prior to today. I am keeping the main account name in use for the time being and have deleted the secondary option until this is rectified.

They appear to have made changes to the login form and have replicated the username code in an iframe (https://account-api.proton.me/challenge/v4/html?Type=0&Name=login), so this HTML code appears twice and the username field in the iframe gets focus and isn't recognized by Firefox. The username field in the form is moved out of view. This makes it impossible for Firefox to fill the username in the iframe via the password manager. I don't know if saved form data is able to fill this input field as both fields have the same #username ID and the field is in a iframe. The password field is placed in a normal position in the DOM and is recognized as the password field (i.e.it has "Use Saved Password" in the context menu).

• https://proton.me/support/update-password-manager

Based on your above reply it looks that if there is one login saved that Firefox fills the first (hidden) field with the username and that is acceptable. Since the input field is not visible and can't be focused you won't be able to select a username from the drop-down list.