Simple Window + Tab Stats

run-this.js

// Run this in your error console
var wc=0, tc=0;
var wm = window.top.opener.Services.wm;
var e = wm.getEnumerator("navigator:browser");
var uc = { };
while (e.hasMoreElements()) {
  var tabs = e.getNext().gBrowser.tabs;
  wc+=1;
  tc += tabs.length;
  Array.forEach(tabs, function(t) {
    var url = t.linkedBrowser.currentURI.spec;
    if (!(url in uc)) {
      uc[url] = 0;
    }
    uc[url]++;
  });
}
var s = "Open windows: " + wc + "\nOpen tabs: " + tc + "\nDuplicated urls:\n";
for ([u,c] in Iterator(uc)) {
  if (c > 1) {
    s += "  " + c + " - " + u + "\n";
  }
}
s;