diff --git a/browser/components/sessionstore/SessionStore.jsm b/browser/components/sessionstore/SessionStore.jsm
index ac192b6274bb..c982395f1f67 100644
--- a/browser/components/sessionstore/SessionStore.jsm
+++ b/browser/components/sessionstore/SessionStore.jsm
@@ -1246,6 +1246,9 @@ var SessionStoreInternal = {
       // we don't want to save the busy state
       delete winData.busy;
 
+      // clear this window from the list, since it has definitely been closed.
+      delete this._windows[aWindow.__SSi];
+
       // Now we have to figure out if this window is worth saving in the _closedWindows
       // Object.
       //
@@ -1291,8 +1294,6 @@ var SessionStoreInternal = {
           this.maybeSaveClosedWindow(winData);
         }
 
-        // clear this window from the list
-        delete this._windows[aWindow.__SSi];
         // Update the tabs data now that we've got the most
         // recent information.
         this.cleanUpWindow(aWindow, winData);