--- a/browser/components/sessionstore/src/SessionStore.jsm
+++ b/browser/components/sessionstore/src/SessionStore.jsm
@@ -3626,16 +3626,25 @@ let SessionStoreInternal = {
     for (let i = oState.windows.length - 1; i >= 0; i--) {
       if (oState.windows[i].isPrivate) {
         oState.windows.splice(i, 1);
         if (oState.selectedWindow >= i) {
           oState.selectedWindow--;
         }
       }
     }
+
+    // Don't save invalid states.
+    // Looks we currently have private windows, only.
+    if (oState.windows.length == 0) {
+      TelemetryStopwatch.cancel("FX_SESSION_RESTORE_COLLECT_DATA_MS");
+      TelemetryStopwatch.cancel("FX_SESSION_RESTORE_COLLECT_DATA_LONGEST_OP_MS");
+      return;
+    }
+
     for (let i = oState._closedWindows.length - 1; i >= 0; i--) {
       if (oState._closedWindows[i].isPrivate) {
         oState._closedWindows.splice(i, 1);
       }
     }
 
 #ifndef XP_MACOSX
     // We want to restore closed windows that are marked with _shouldRestore.