Firefox4+Scriptitshで動かなかったので修正

ldr favicon hack.user.js

// ==UserScript==
// @name           ldr favicon hack
// @namespace      http://d.hatena.ne.jp/javascripter/
// @include        http://reader.livedoor.com/reader/
// ==/UserScript==
(function() {
    // 未読数の数値が変化したら更新
    document.getElementById("total_unread_count").addEventListener("DOMAttrModified", function(e) {
        updateFavicon();
    }, true);
    var favicon = document.evaluate('//head/link[@rel="shortcut icon"]', document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
    var canvas = document.createElement('canvas');
    var head = document.getElementsByTagName('head')[0];

    function updateFavicon() {
        var unread = unsafeWindow.subs.model.get_unread_count();
        canvas.width = canvas.height = 16;
        with (canvas.getContext('2d')) {
            clearRect(0, 0, 16, 16);
            mozTextStyle = 22 + (unread.toString().length) * -4 + "px sans-serif";
            translate(0, 14);
            mozDrawText(unread);
        }
        favicon.href = canvas.toDataURL();
        head.appendChild(favicon);

    }
})();