It would be helpful to know your domain name because we would probably see immediately what the problem is.
When the browser shows an indication that a page is not secure, you can usually click on that icon to have the browser show you more details about the problem.
This is often a problem with mixed content, where secure and insecure resources are combined on the same page. You can diagnose that with
It’s true that since that version of Firefox, your site needs HTTPS in order to not display a warning on password inputs. Do you have a Let’s Encrypt certificate for your site that you’re having trouble using somehow?