I scrolled down and still couldn't see the checkboxes under the last row. Maybe it also doesn't work on Firefox, because as trivial as it is to make a site that works everywhere no one does. Normally this frustrates me, but if that's the case here it kind of makes sense.
I did exactly that as well. I’m also happy that there appear to be several different captchas, as others mentioned ones I didn’t see. The best one I clicked through was “select every picture with glasses” and every picture had either eyeglasses, drinking glasses or panes of glass.