It is easy to take screenshots on Firefox OS. Here's how:
Your screenshot will be stored in the Gallery app where you can edit and share it. For more, see Manage photo and video gallery.
In Firefox OS 2.1 (on ZTE Open C), take a screen capture works only with volume down + power button.
Why this article does not refer to this ?
I agree with Klev. I've been testing the flame and the instructions given in the article don't apply to it. Do we want to break down the instructions to the family devices -> generic instructions separate?