Edit in JSFiddle

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.fillText('Hi there.', 50, 50);

canvas.mozPrintCallback = function(printState) {
  var printCtx = printState.context;
  printCtx.fillText('I\'m only visible when printed.', 50, 50);
  printState.done();
};