--- a/browser/components/sessionstore/SessionStore.jsm
+++ b/browser/components/sessionstore/SessionStore.jsm
@@ -1483,32 +1483,43 @@ var SessionStoreInternal = {
    *        total (int):
    *          The total number of windows to be flushed.
    *        current (int):
    *          The current window that we're waiting for a flush on.
    *
    * @return Promise
    */
   flushAllWindowsAsync: Task.async(function*(progress={}) {
-    let windowPromises = [];
+    let windowPromises = new Map();
     // We collect flush promises and close each window immediately so that
     // the user can't start changing any window state while we're waiting
     // for the flushes to finish.
     this._forEachBrowserWindow((win) => {
-      windowPromises.push(TabStateFlusher.flushWindow(win));
-      win.close();
+      windowPromises.set(win, TabStateFlusher.flushWindow(win));
+
+      // We have to wait for these messages to come up from
+      // each window and each browser. In the meantime, hide
+      // the windows to improve perceived shutdown speed.
+      let baseWin = win.QueryInterface(Ci.nsIInterfaceRequestor)
+                       .getInterface(Ci.nsIDocShell)
+                       .QueryInterface(Ci.nsIDocShellTreeItem)
+                       .treeOwner
+                       .QueryInterface(Ci.nsIBaseWindow);
+      baseWin.visibility = false;
     });
 
-    progress.total = windowPromises.length;
+    progress.total = windowPromises.size;
+    progress.current = 0;
 
     // We'll iterate through the Promise array, yielding each one, so as to
     // provide useful progress information to AsyncShutdown.
-    for (let i = 0; i < windowPromises.length; ++i) {
-      progress.current = i;
-      yield windowPromises[i];
+    for (let [win, promise] of windowPromises) {
+      yield promise;
+      this._collectWindowData(win);
+      progress.current++;
     };
 
     // We must cache this because _getMostRecentBrowserWindow will always
     // return null by the time quit-application occurs.
     var activeWindow = this._getMostRecentBrowserWindow();
     if (activeWindow)
       this.activeWindowSSiCache = activeWindow.__SSi || "";
     DirtyWindows.clear();