miya2000さんの http://github.com/miya2000/user.js/tree/master/nicovideo_view_without_login.js をFirefox用に変更したもの.

nico.user.js

// ==UserScript==
// @name nicovideo - view without login.
// @namespace http://d.hatena.ne.jp/miya2000/
// @author miya2000
// @version 1.1.0
// @include http://www.nicovideo.jp/watch/*
// @exclude http://*http*
// a little modified for Firefox.
// ==/UserScript==
(function(win){
  var doc = win.document;
  var bak_write = doc.write;
  function main(){
    var page = win.$('PAGEBODY');
    win.removeEventListener('load', main, false);
    if (win.User && win.User.id) return;
    var video_id = location.href.match(/[^/]*$/);
    if (!video_id) return;
    function createDocumentFragmentByString(html){
      var range = doc.createRange();
      range.selectNode(doc.body);
      return range.createContextualFragment(html);
    }
    var thumb_script = doc.createElement('script');
    thumb_script.src = 'http://www.nicovideo.jp/thumb_watch/' + video_id;
    thumb_script.style.display = 'none';
    doc.write = function(html){
      doc.write = bak_write;
      var node = createDocumentFragmentByString(html).firstChild;
      page.insertBefore(node, page.firstChild);
    };
    win.Element.setStyle('PAGEBODY',{'text-align':'center'});
    doc.body.appendChild(thumb_script);
  }
  win.addEventListener('load', main, false);
})(this.unsafeWindow || this);