gBrowser.addEventListener("DOMContentLoaded",blank,false);

function blank(){
　var tab = gBrowser.selectedTab;
　
　if (tab.linkedBrowser.currentURI.spec == "about:blank"  ) {
　　alert("About:Blank!!");
　}
}

window.addEventListener('load', function() {
  window.removeEventListener('load', arguments.callee, false);

  gBrowser.addEventListener("DOMContentLoaded",onTabLoad,false);
}, false);

function onTabLoad(aEvent){
  var doc = aEvent.originalTarget.defaultView.top.document;
  var tabs = Array.slice(gBrowser.mTabContainer.childNodes);
  var loadedTab;
  for each (var tab in tabs)
  {
    if (tab.linkedBrowser.contentDocument != doc) continue;
    loadedTab = tab;
    break;
  }
　if (loadedTab.linkedBrowser.currentURI.spec == "about:blank"  ) {
 　alert("About:Blank!!");
 }
}