mosuehunt_horn_bot.user.js

// ==UserScript==
// @name           MouseHunt Horn bot
// @namespace      adachinko
// @include        http://apps.facebook.com/mousehunt/index.php
// @include        http://apps.facebook.com/mousehunt/
// ==/UserScript==

(function() {
function getFirstElementByXPath(xpath, node) {
    var node = node || document
    var doc = node.ownerDocument ? node.ownerDocument : node
    var result = doc.evaluate(xpath, node, null,
        XPathResult.FIRST_ORDERED_NODE_TYPE, null)
    return result.singleNodeValue ? result.singleNodeValue : null
}

getComputedStyle = document.defaultView && document.defaultView.getComputedStyle;

function sound_the_horn() {
  var puzzle = getFirstElementByXPath('//form[@class="puzzleform"]');
  if(puzzle) {
    GM_log('puzzle found. ');
    alert('puzzle found');
    return;
  }

  GM_log('check the horn.');
  var horn = getFirstElementByXPath('//*[@class="hornarea"]/a')

  if(horn) {
    // check hidden
		var defaultView = horn.ownerDocument.defaultView;

		if(!defaultView)
      return

		var computedStyle = defaultView.getComputedStyle(horn, null);
    
    if(computedStyle.display == 'none') {
      GM_log('horn is hidden');
      return;
    }
    
    GM_log('sound the horn');
    var clickEvent = window.document.createEvent("MouseEvent"); 
    clickEvent.initEvent("click", false, true); 
    horn.dispatchEvent(clickEvent);
  }
}

window.addEventListener('load', function() {
  sound_the_horn();
  setInterval(sound_the_horn, 30 * 1000);
}, false);  

})();