为什么Firefox反向重放css动画？

Question

如下面的编码器所示，导航链接按照应有的方式淡入，但是当它们悬停时，它们似乎反向重放动画，然后再快速转发。这真的很奇怪。 http://codepen.io/mildrenben/pen/wBMVdm

<header>
  <h1>BEN MILDREN</h1>
  <p>Designer and Developer</p>
</header>
<nav>
  <a>About</a>
  <a>Portfolio</a>
  <a>Blog</a>
  <a>Contact</a>
</nav>




<link href='http://fonts.googleapis.com/css?family=Open+Sans:400,600,300,700,800|Gentium+Book+Basic:400,400italic,700italic' rel='stylesheet' type='text/css'>

CSS：

html, body {
  width: 100%;
  height: 100%;
}

body {
  background: url(https://ununsplash.imgix.net/reserve/D9xlw7UxTBqQw5sLf8cJ_reef%20insp-72.jpg?q=75&fm=jpg&s=a46f72e373f7af2b3bba993f565e24be);
  background-size: cover;
}

* {
  margin: 0;
}

$color: #4C5C58;
$color-light: #5B6B67;

header {
  position: absolute;
  top: 27%;
  left: calc(50% - 182px);
  text-align: center;
  -webkit-animation: anim2 1s ease-in-out 2.5s forwards;
}

@-webkit-keyframes anim1 {
  100% {
    transform: translateY(-12px);
    opacity: 1;
  }
}

@-webkit-keyframes anim2 {
  100% {
    transform: translateY(-70px);
  }
}

@keyframes anim1 {
  100% {
    transform: translateY(-12px);
    opacity: 1;
  }
}

@keyframes anim2 {
  100% {
    transform: translateY(-70px);
  }
}

h1 {
  font-family: 'Open Sans', sans-serif;
  font-size: 56px;
  font-weight: 800;
  color: $color;
  opacity: 0;
  -webkit-animation: anim1 1.5s ease-in-out forwards;
  animation: anim1 1.5s ease-in-out forwards;
}

p {
  font-family: 'Gentium Book Basic', serif;
  font-style: italic;
  font-size: 20px;
  color: $color-light;
  opacity: 0;
  -webkit-animation: anim1 1.5s ease-in-out 0.3s forwards;
}

nav {
  position: absolute;
  left: calc(50% - 183px);
  top: 45%;
}

a {
  height: 40px;
  width: auto;
  background: none;
  margin: 0 20px;
  border: none;
  cursor: pointer;
  font-family: 'Gentium Book Basic', sans-serif;
  font-weight: 500;
  font-size: 15px;
  color: $color-light;
  transition: all 0.3s ease-in-out;
  line-height: 50px;
  opacity: 0;
  display: inline-block;
}

a:hover {
  border-bottom: double 3px $color-light;
}

img {
  width: 150px;
  position: absolute;
  margin-left: 72px;
  top: 38px;
}

nav p {
  font-style: italic;
  font-size: 14px;
  position: absolute;
  top: 80px;
  left: 100px;
}

a:nth-of-type(1) {
  -webkit-animation: anim1 1s ease-in-out 3s forwards;
 animation: anim1 1s ease-in-out 3s forwards;
}

a:nth-of-type(2) {
  -webkit-animation: anim1 1s ease-in-out 3.1s forwards;
  animation: anim1 1s ease-in-out 3.1s forwards;
}

a:nth-of-type(3) {
  -webkit-animation: anim1 1s ease-in-out 3.2s forwards;
  animation: anim1 1s ease-in-out 3.2s forwards;
}

a:nth-of-type(4) {
  -webkit-animation: anim1 1s ease-in-out 3.3s forwards;
  animation: anim1 1s ease-in-out 3.3s forwards;
}