How to output only the first instance of a Matrix block type field
You can either use external bool variable

{% set firstNbContentDisplayed = false %}
{% for block in entry.newsBlock %}
    {% switch block.type %}
        {% case 'nbTitle' %}
            ...
        {% case 'nbContent' %}
            {% if not firstNbContentDisplayed %} 
                {% set firstNbContentDisplayed = true %}
                ...
            {% endif %}
    {% endswitch %}
{% endfor %}


or if possible in your case separate nbContent and limit it to 1

{% for block in entry.newsBlock %}
    {% switch block.type %}
        {% case 'nbTitle' %}
            ...
    {% endswitch %}
{% endfor %}

{% for block in entry.newsBlock.type('nbContent').limit(1) %}
    ...
{% endfor %}