Display: flex and Position: absolute conflict

Question

Here's a screenshot of the problem.
To bottom: 1 - Chrome, 2 - Opera, 3 - Firefox, 4 - Edge.

Below I have written executable code.
For convenience, here codepen.

The basic idea: Gallery label and logo in the center position, and the rest - on the sides.
Transform will be used for animation.

Beyond measure I am grateful if you help to understand how to maintain flex in Firefox.
More precisely, I think that align-items: center is not correctly displayed in Firefox. Please help.

.gallerypage {
  margin: 0;
  padding: 0;
  background: #fff;
}

.gallerypage .header {
  position: relative;
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-flex-flow: row nowrap;
      -ms-flex-flow: row nowrap;
          flex-flow: row nowrap;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
      -ms-flex-pack: center;
          justify-content: center;
  -webkit-box-align: center;
  -webkit-align-items: center;
      -ms-flex-align: center;
          align-items: center;
  width: 100%;
  height: 100px;
  background: gray;
}

.gallerypage .header a {
  cursor: pointer;
  position: absolute;
  -webkit-flex-shrink: 0;
      -ms-flex-negative: 0;
          flex-shrink: 0;
  text-align: center;
  white-space: pre-wrap;
  text-indent: -6px;
  line-height: 30px;
  font-family: "Kaushan Script", cursive;
  font-size: 30px;
}

.gallerypage .header .sketches {
  background-color: #fff;
  width: 110px;
  -webkit-transform: translate(-400px, 0px);
      -ms-transform: translate(-400px, 0px);
          transform: translate(-400px, 0px);
}

.gallerypage .header .smth {
  background-color: #fff;
  width: 140px;
  height: 60px;
  -webkit-transform: translate(-200px, 0px);
      -ms-transform: translate(-200px, 0px);
          transform: translate(-200px, 0px);
}

.gallerypage .header .gallery {
  position: static;
  text-indent: -14px;
  line-height: normal;
  width: 185px;
  height: 90px;
  background: gold;
  font-size: 60px;
}

.gallerypage .header .logo {
  position: static;
  width: 60px;
  height: 80px;
  background: hotpink;
}

.gallerypage .header .logo .vs-logo {
  display: block;
  -webkit-flex-shrink: 0;
      -ms-flex-negative: 0;
          flex-shrink: 0;
  width: 60px;
  height: 80px;
}

.gallerypage .header .blog {
  background-color: #fff;
  width: 60px;
  height: 38px;
  -webkit-transform: translate(200px, 0px);
      -ms-transform: translate(200px, 0px);
          transform: translate(200px, 0px);
}

.gallerypage .header .about {
  white-space: pre-wrap;
  background-color: #fff;
  width: 100px;
  -webkit-transform: translate(400px, 0px);
      -ms-transform: translate(400px, 0px);
          transform: translate(400px, 0px);
}

<div class="gallerypage">
  <div class="header">
    <a class="sketches">Sketches</a>
    <a class="smth">Something serious</a>
    <a class="gallery">Gallery</a><a class="logo"></a>
    <a class="blog">Blog</a><a class="about">About it</a>
  </div>
/<div>

Answer 1

Both vertical and horizontal alignment issues are resolved in Firefox. I've also applied a simple animation to make sure it's compatible with what you're trying to do (hover on header to transition the other items).

Link: http://codepen.io/anon/pen/YNXNLN

To fix the vertical center alignment on firefox, add this style (I mean merge, since you already use transform attribute):

a {
 transform: translateY(-50%);
}

But then it will be positioned at the very top on Chrome, so you'll add this style to fix it:

a {
 top: 50%;
}

Same will be done with horizontal alignment "left: 50%" and you already use translateX, so the value will be up to you.