Firefox Animation Issues - Clickable Overlay

I've run into some issues trying to animate the clickable overfly in firefox.

I started with this example http://codepen.io/ncerminara/pen/EdaBm

Everything works well except for the fadeIn on the overlay. Tested in Safari and Chrome and works fine.

Haven't found a solution yet.

Here's the altered css I used:

.overlay {
  // Overlay base styles
  position: fixed;
  background: rgba(0, 0, 0, 0.75);
  width: 100%;
  height: 100%;
  display: none;
  z-index: 999999;
  cursor: pointer;

  // Set transition for overlay
  -webkit-transition:         all 425ms ease;
  -moz-transition:            all 425ms ease;
  -ms-transition:             all 425ms ease;
  -o-transition:              all 425ms ease;
  transition:                 all 425ms ease;


  // Set the animation duration
  -webkit-animation-duration:   1.1s;
  -moz-animation-duration:      1.1s;
  -ms-animation-duration:       1.1s;
  -o-animation-duration:        1.1s;
  animation-duration:           1.1s;

  // Set the animation fill mode
  -webkit-animation-fill-mode:  both;
  -moz-animation-fill-mode:     both;
  -ms-animation-fill-mode:      both;
  -o-animation-fill-mode:       both;
  animation-fill-mode:          both;

  // Name the Animation
  -webkit-animation-name:       fadeIn;
  -moz-animation-name:          fadeIn;
  -ms-animation-name:           fadeIn;
  -o-animation-name:            fadeIn;
  animation-name:               fadeIn;
}

// When is show class is applied
// made the overlay display block
.scotch-is-showing .overlay {
  display: block;
}

// Setup keyframes animations
// Chrome
@-webkit-keyframes fadeIn {
  0%   { opacity: 0; }
  100% { opacity: 1; }
}

// Firefox
@-moz-keyframes fadeIn {
  0%   { opacity: 0; }
  100% { opacity: 1; }
}

// Opera
@-o-keyframes fadeIn {
  0%   { opacity: 0; }
  100% { opacity: 1; }
}

// All other browsers
@keyframes fadeIn {
  0%   { opacity: 0; }
  100% { opacity: 1; }
}