gistfile1.txt

  getScreenshot: (tab) => {
    Task.spawn(function* () {
      let win = tab.window;
      let dpr = win.devicePixelRatio;
      let canvas = win.document.createElement("canvas");
      let ctx = canvas.getContext("2d");
      // Grab the visible viewport coordinates
      let x = win.document.documentElement.scrollLeft;
      let y = win.document.documentElement.scrollTop;
      let w = win.innerWidth;
      let h = win.innerHeight;
      // Scale according to devicePixelRatio
      canvas.width = dpr * w;
      canvas.height = dpr * h;
      ctx.scale(dpr, dpr);
      ctx.drawWindow(win, x, y, w, h, '#ffffff');
      return yield canvas.toDataURL();
      // Test on the Nexus 6P, because that has a crazy screen.
    });
  },
  
  [...]
  
  let data = this.getScreenshot(selectedTab);
  Services.console.logStringMessage('~~~~' + data);