SVG background in firefox

Question

Working on my friend's website and I can't figure out how to display the SVG right in Firefox. In Edge it also disappears while I'm resizing the browser.

http://lene.isreborn.com/

CSS - (I have to use important because it's Wordpress):

header::after {
    content:"";
    display: block;
    background: url(/svg-filer/header-background.svg);
    background-size: contain;
    background-repeat: no-repeat;
    top: 0;
    left:-1%;
    width: 102% !important;
    height: 90px !important;
    position: absolute;
    z-index: -1;

    -ms-transition: top ease .5s;
    -moz-transition: top ease .5s;
    -webkit-transition: top ease .5s;
    transition: top ease .5s;
}

header.sticky-active::after { 
    top: -20px;
}

SVG File:

  <?xml version="1.0" encoding="utf-8"?>
    <!-- Generator: Adobe Illustrator 19.0.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
    <svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"
         width="100%" height="90px" viewBox="0 0 1920 90" preserveAspectRatio="none">    
    <polygon id="XMLID_3_" fill="#FFFF00" points="1,99.7 1399.2,130 1921,99.7 1921,0 1,0 "/>
    </svg>

Answer 1

I've got it right now: The background-size does not work with 'contain' but with 100% 100%, See Mozilla Background rules
Link to developer site: See here

header::after {
    content:"";
    display: block;
    background: url(/svg-filer/header-background.svg);
    background-size: 100% 100% !important;
    background-repeat: no-repeat;
    top: 0;
    left:-1%;
    width: 102% !important;
    height: 90px !important;
    position: absolute;
    z-index: -1;

    -ms-transition: top ease .5s;
    -moz-transition: top ease .5s;
    -webkit-transition: top ease .5s;
    transition: top ease .5s;
}

header.sticky-active::after { 
    top: -20px;
}