Should be like so:

```html
<p>{{ _('Visit <a href="{url}">support.mozilla.com</a> for more help!')|f(url='http://support.mozilla.com/') }}</p> 
```

Or:

```html
<p>
  {% trans url='http://support.mozilla.com/' %}
    Visit <a href="{{ url }}">support.mozilla.com</a> for more help!
  {% endtrans %}
</p>
```