In addition to the answer supplied by @filippo-lauria, you should check for the existence of errors:

...
 <label for="{{ form.firstname.id_for_label }}">First Name:</label>
   {{ form.firstname }}
   {% if form.firstname.errors %}
       {{ form.firstname.errors }}
   {% endif %}
 ...

As of Django 5.1, you will get template errors without this check if errors don't exist.