{% load jmbo_inclusion_tags foundry_tags %}

{% if object_list.exists %}
    <div class="foundry-container foundry-listing-promo">
        <div class="items">
        <ul id="slider">
        {% for object in object_list %}
            <li>
                <img src="{{ object.get_listing_promo_LAYER_url }}" alt="{{ object.title }}" alt="{{ object.title }}" />
                <div class="info">
                    {% if object.categories.all.count %}<div class="subtitle">{{ object.categories.all|join_titles }}:</div>{% endif %}
                    <div class="title"><a href="{{ object.get_absolute_url }}">{{ object.title }}</a></div>
                    <div class="description">{{ object.description }}</div>
                </div>
            </li>
        {% endfor %}
        </ul>
        </div>
    </div>        
{% endif %}

{% load jmbo_inclusion_tags foundry_tags %}

{% if object_list.exists %}
    <div class="foundry-container foundry-listing-promo">
        <div class="items">
        <ul id="slider">
        {% for object in object_list %}
            <li>
                <img src="{{ object.get_listing_promo_LAYER_url }}" alt="{{ object.title }}" alt="{{ object.title }}" />
                <div class="info">
                    {% if object.categories.all.count %}<div class="subtitle">{{ object.categories.all|join_titles }}:</div>{% endif %}
                    <div class="title"><a href="{{ object.as_leaf_class.get_absolute_url }}">{{ object.title }}</a></div>
                    <div class="description">{{ object.description }}</div>
                </div>
            </li>
        {% endfor %}
        </ul>
        </div>
    </div>        
{% endif %}

Use get_absolute_url from leaf class
Use get_absolute_url from leaf class
