diff --git a/browser/base/content/browser-tabPreviews.js b/browser/base/content/browser-tabPreviews.js
index 5ba9a2620aec..785101305f93 100644
--- a/browser/base/content/browser-tabPreviews.js
+++ b/browser/base/content/browser-tabPreviews.js
@@ -310,12 +310,14 @@ var ctrlTab = {
   },
 
   advanceFocus: function ctrlTab_advanceFocus(aForward) {
-    if (this.panel.state == "open") {
+    if (this._selectedIndex == -1) {
+      // No virtual selectedIndex, focus must be in the panel already.
       if (aForward)
         document.commandDispatcher.advanceFocus();
       else
         document.commandDispatcher.rewindFocus();
     } else {
+      // Focus isn't in the panel yet, so we maintain a virtual selectedIndex.
       do {
         this._selectedIndex += aForward ? 1 : -1;
         if (this._selectedIndex < 0)