Skrollr.js Background attachment issue

Looking to see if anyone could help me out. I created this pen:

.. I am using the Skrollr.js library ( I am tying to achieve an effect very similar to this: (Create together section), where the frame when scroll by locks the screen in place and stops at the bottom on the last frame.

seems like Firefox and Chrome/Safari handle background-attachment: fixed; differently (chrome giving more desired output) ... any ideas would be great.. been, been wracking my brain for a while. After I get it figured out standalone I need to port in to Wordpress, so hoping someone could help me understand why this isn't functioning quite right.