Snapshot a page in Firefox

snap_screen.js

// Current window
var el = window.document.createElementNS("http://www.w3.org/1999/xhtml", "canvas");
el.width = window.innerWidth;
el.height = window.innerHeight;
var ctx = el.getContext("2d");
SpecialPowers.wrap(ctx).drawWindow(window, 0, 0, window.innerWidth, window.innerHeight, "rgba(0,0,0,0)",
  ctx.DRAWWINDOW_USE_WIDGET_LAYERS | ctx.DRAWWINDOW_DRAW_VIEW | ctx.DRAWWINDOW_DRAW_CARET);
dump("\n" + el.toDataURL() + "\n");

// Parent window in e10s mode
function getSnapshot() {
  function parentProcessSnapshot() {
    addMessageListener('snapshot', function() {
      Components.utils.import('resource://gre/modules/Services.jsm');
      var topWin = Services.wm.getMostRecentWindow('navigator:browser');
      // take the snapshot
      var canvas = topWin.document.createElementNS("http://www.w3.org/1999/xhtml", "canvas");
      canvas.width = topWin.innerWidth;
      canvas.height = topWin.innerHeight;
      var ctx = canvas.getContext("2d");
      ctx.drawWindow(topWin, 0, 0, topWin.innerWidth, topWin.innerHeight, 'rgb(255,255,255)', ctx.DRAWWINDOW_DRAW_VIEW | ctx.DRAWWINDOW_USE_WIDGET_LAYERS | ctx.DRAWWINDOW_DRAW_CARET);
      return canvas.toDataURL();
    });
  }

  if (typeof getSnapshot.chromeHelper == 'undefined') {
    // This is the first time getSnapshot is being called; do initialization
    getSnapshot.chromeHelper = SpecialPowers.loadChromeScript(parentProcessSnapshot);
    SimpleTest.registerCleanupFunction(function() { getSnapshot.chromeHelper.destroy() });
  }

  return getSnapshot.chromeHelper.sendSyncMessage('snapshot').toString();
}

dump(">>> SNAPSHOT: \n");
dump('\n\n>>> ' + getSnapshot() + '\n\n');