hotfix for newly-discovered browser_526613.js failure; will file a bug on this

browser/components/sessionstore/src/nsSessionStore.js

@@ -1734,6 +1734,8 @@ SessionStoreService.prototype = {
     var nonPopupCount = 0;
     var ix;
     for (ix in this._windows) {
+      if (this._windows[ix].tabs.length == 0)
+        continue;
       total.push(this._windows[ix]);
       windows.push(ix);
       if (!this._windows[ix].isPopup)