Is Firefox presenting an error page, or do you mean there is a little drop-down message about insecure logins?

I don't know why an important form presented by a legitimate company would have either of those problems...

Error page

Does Firefox show a specific technical code like SEC_ERROR_SOMETHING? You may need to click an Advanced button in the error page to find the code.

Spoiler alert: I look them up in this article: How to troubleshoot security error codes on secure websites.

Notice on login form

Usually you can remove the drop-down temporarily by tapping the Esc key. But if the message is not logical for the data requested, beware of a data stealer (possible fake form).

Does that website only work with the insecure HTTP connection?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot