Since this question is not about my `ssh` problem but about special steps for RasPi 3, I think I found the answer. In [this question][1] I stumbled over the topic `rpi-update`. Thus, for Raspbian, the following steps updated my firmware:

    # apt-get install rpi-update
    # rpi-update
    # shutdown -r now

Sadly, this didn't solve my `ssh` problem; but that was not the question.


  [1]: http://raspberrypi.stackexchange.com/q/27723/12333