Fix a bug about context menu on Firefox 23.0a2+
Fix a bug about Preference on Firefox 22.0+

FYI:
  https://bugzilla.mozilla.org/show_bug.cgi?id=407956
  https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsITreeView#getCellProperties()
  http://moz-addon.g.hatena.ne.jp/teramako/20130306/1362573270
  http://k9q.hateblo.jp/entry/2013/04/18/121126
Merge pull request #58 from tombfix/pr3-fix

Fix 2 bugs about Firefox 22+