Look in /etc/httpd/conf/vhosts.d/ipfire-interface-ssl.conf There is “Require user admin” in various places. You could make changes to “Require user alice”.
There is also /var/ipfire/auth/users that contains admin:pw. You can save that file as users.orig and create a new users file that will have alice:pw (see htpasswd utility).
If all goes well, https://ipfire_address:444/ should auth as alice and password you gave.
As long as you are using a secure password, you should be fine. But those files are part of the IPFire system configuration and will be overwritten in any of the next updates.