UPDATE: This feature has been removed from Firefox if you are not writing a plugin. Please find various alternatives, see http://badassjs.com/post/12473322192/hack-of-the-day-rendering-html-to-a-canvas and https://developer.mozilla.org/en- US / docs / Web / API / CanvasRenderingContext2D / drawWindow
In those days, you could: take a screenshot of the firefox window, yes, and it was so easy that it hurt. You should have done it on canvas. See https://developer.mozilla.org/en/drawing_graphics_with_canvas#Rendering_Web_Content_Into_A_Canvas
<canvas id='my-canvas'></canvas>
<script>
var canvas = document.getElementById('my-canvas');
var ctx = canvas.getContext("2d");
ctx.drawWindow(window, 0,0, 100, 200, "rgb(255,255,255)");
open(canvas.toDataURL("image/png"));
</script>
If you mean a screenshot of the entire desktop, I don’t think that
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