screenshot_firefox.js

function captureVisibleTab() {
  var win = require("window-utils").windowIterator().next(); // the actual firefox chrome (not the browser chrome.. the firefox concept)
  var {Cc, Ci} = require("chrome");

  // the... active window...
  var mainWindow = win.QueryInterface(Ci.nsIInterfaceRequestor)
                       .getInterface(Ci.nsIWebNavigation)
                       .QueryInterface(Ci.nsIDocShellTreeItem)
                       .rootTreeItem
                       .QueryInterface(Ci.nsIInterfaceRequestor)
                       .getInterface(Ci.nsIDOMWindow);

  var contentWindow = mainWindow.gBrowser.getBrowserForTab(mainWindow.gBrowser.selectedTab).contentWindow;
  var contentDocument = contentWindow.document;
  var clientInfo = contentDocument.compatMode == "CSS1Compat" ?
    contentDocument.documentElement : contentDocument.body;

  var backgroundColor = '#000000'; // requestDetails.backgroundColor;
  var width = contentWindow.innerWidth;
  var height = contentWindow.innerHeight;

  var region = {
    x: contentWindow.scrollX,
    y: contentWindow.scrollY,
    width: clientInfo.clientWidth,
    height: clientInfo.clientHeight
  };

  var canvas = contentDocument.createElement('canvas');
  canvas.style.maxwidth = region.width + "px";
  canvas.style.maxheight = region.height + "px";
  canvas.width = region.width;
  canvas.height = region.height;

  var ctx = canvas.getContext('2d');
  ctx.clearRect(region.x, region.y, region.width, region.height);
  try {
    ctx.drawWindow(contentWindow, region.x, region.y, region.width, region.height, backgroundColor);
  } catch(err) {
    console.error(JSON.stringify(err)); // firefox is pretty unfriendly to extnesion devs..
    return;
  } // sorry :(

  return canvas.toDataURL();
}