don't override single tab when pref is 'append'

browser/components/bookmarks/content/bookmarks.js

@@ -573,12 +573,12 @@ var BookmarksCommand = {
       if (doReplace)
         index0 = 0;
       else {
-        for (index0=tabCount-1; index0>0; --index0)
-          if (browser.browsers[index0].webNavigation.currentURI.spec != "about:blank") {
-            ++index0;
+        for (index0=tabCount-1; index0>=0; --index0)
+          if (browser.browsers[index0].webNavigation.currentURI.spec != "about:blank")
             break;
-          }
+        ++index0;
       }
+
       var index  = index0;
       while (containerChildren.hasMoreElements()) {
         var res = containerChildren.getNext().QueryInterface(kRDFRSCIID);