--- a/browser/modules/BrowserUITelemetry.jsm
+++ b/browser/modules/BrowserUITelemetry.jsm
@@ -179,17 +179,27 @@ this.BrowserUITelemetry = {
     let buttonKey = BUTTONS[aButton];
     if (buttonKey) {
       let countObject =
         this._ensureObjectChain([aCategory, aAction, buttonKey], 0);
       countObject[buttonKey]++;
     }
   },
 
+  _firstWindowMeasurements: null,
   _registerWindow: function(aWindow) {
+    // We'll gather measurements on the first non-popup window that opens
+    // after it has painted. We do this here instead of waiting for
+    // UITelemetry to ask for our measurements because at that point
+    // all browser windows have probably been closed, since the vast
+    // majority of saved-session pings are gathered during shutdown.
+    if (!this._firstWindowMeasurements && aWindow.toolbar.visible) {
+      this._firstWindowMeasurements = this._getWindowMeasurements(aWindow);
+    }
+
     aWindow.addEventListener("unload", this);
     let document = aWindow.document;
 
     for (let areaID of CustomizableUI.areas) {
       let areaNode = document.getElementById(areaID);
       if (areaNode) {
         (areaNode.customizationTarget || areaNode).addEventListener("mouseup", this);
       }
@@ -306,30 +316,18 @@ this.BrowserUITelemetry = {
     // being clicked.
     if (ALL_BUILTIN_ITEMS.indexOf(item.id) != -1) {
       // Base case - we clicked directly on one of our built-in items,
       // and we can go ahead and register that click.
       this._countMouseUpEvent("click-builtin-item", item.id, aEvent.button);
     }
   },
 
-  getToolbarMeasures: function() {
-    // Grab the most recent non-popup, non-private browser window for us to
-    // analyze the toolbars in...
-    let win = RecentWindow.getMostRecentBrowserWindow({
-      private: false,
-      allowPopups: false
-    });
-
-    // If there are no such windows, we're out of luck. :(
-    if (!win) {
-      return {};
-    }
-
-    let document = win.document;
+  _getWindowMeasurements: function(aWindow) {
+    let document = aWindow.document;
     let result = {};
 
     // Determine if the Bookmarks bar is currently visible
     let bookmarksBar = document.getElementById("PersonalToolbar");
     result.bookmarksBarEnabled = bookmarksBar && !bookmarksBar.collapsed;
 
     // Examine all customizable areas and see what default items
     // are present and missing.
@@ -359,27 +357,31 @@ this.BrowserUITelemetry = {
         }
         // else, it's provided by an add-on, and we won't record it.
       }
     }
 
     // Now go through the items in the palette to see what default
     // items are in there.
     let paletteItems =
-      CustomizableUI.getUnusedWidgets(win.gNavToolbox.palette);
+      CustomizableUI.getUnusedWidgets(aWindow.gNavToolbox.palette);
     let defaultRemoved = [item.id for (item of paletteItems)
                           if (DEFAULT_ITEMS.indexOf(item.id) != -1)];
 
     result.defaultKept = defaultKept;
     result.defaultMoved = defaultMoved;
     result.nondefaultAdded = nondefaultAdded;
     result.defaultRemoved = defaultRemoved;
 
+    return result;
+  },
+
+  getToolbarMeasures: function() {
+    let result = this._firstWindowMeasurements || {};
     result.countableEvents = this._countableEvents;
-
     return result;
   },
 };
 
 /**
  * Returns the id of the first ancestor of aNode that has an id. If aNode
  * has no parent, or no ancestor has an id, returns null.
  *