Firefox OS app icon underlay as provided by Mozilla at http://www.mozilla.org/en-US/styleguide/products/firefox-os/icons/

underlay.svg

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg
   xmlns="http://www.w3.org/2000/svg" version="1.1"
   xmlns:xlink="http://www.w3.org/1999/xlink"
   width="600"
   height="600">
  <defs>
    <symbol id="appLogoUnderlay" viewBox="0 0 600 600">
        <linearGradient
           id="diagonalAlpha"
           x1="257" y1="586.84"
           x2="514.66" y2="105"
           gradientUnits="userSpaceOnUse">
          <stop
             style="stop-color:#000000;stop-opacity:0.341;"
             offset="0" />
          <stop
             style="stop-color:#000000;stop-opacity:0.078;"
             offset="1" />
        </linearGradient>
        <linearGradient
           id="strokeGrad"
           x1="50" y1="50"
           x2="550" y2="550"
           gradientUnits="userSpaceOnUse">
          <stop
             style="stop-color:#777777;stop-opacity:1;"
             offset="0" />
          <stop
             style="stop-color:#BBBBBB;stop-opacity:1;"
             offset="1" />
        </linearGradient>
        <filter id="f1" x="0" y="0">
          <feGaussianBlur in="SourceGraphic" stdDeviation="1.5" />
        </filter>
        <circle
           cx="300" cy="300" r="290"
           filter="url(#f1)"
           style="fill:#777777;stroke:url(#strokeGrad);stroke-width:9;fill-opacity:1" />
        <path
           style="fill:url(#diagonalAlpha);stroke:none;fill-opacity:1" 
           d="M 257,586.84 A 323,323 0 0 1 514.66,105 A 290,290 0 0 1 257,586.84 Z"
           id="path3831" />
    </symbol>
  </defs>
  <use xlink:href="#appLogoUnderlay" x="10" y="10" width="580" height="580" />
</svg>