Text position on canvas is different in FireFox and Chrome

I need to draw a text string at a precise position on HTML5 canvas.

Here's my test code:

<!DOCTYPE html>
<html>
<body>
<canvas id="mainCanvas" width="320" height="240" style = "border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
   window.onload = function() {
      var canvas = document.getElementById("mainCanvas");
      var ctx = canvas.getContext("2d");      
      ctx.textBaseline = "top";
      ctx.font = '100px Arial';
      ctx.textAlign = 'left';
      ctx.fillStyle = 'rgba(0, 0, 0, 255)';
      ctx.fillText('Test', 0, 0);
   }
</script>
</body>
</html>

The margin at the top is different in Chrome and Firefox:

I'm going to draw other elements (e.g. images, shapes) on the canvas, and I need to make sure the text appears at the same position in all browsers. Is it possible?