--- a/browser/base/content/test/browser_CTP_drag_drop.js
+++ b/browser/base/content/test/browser_CTP_drag_drop.js
@@ -127,17 +127,20 @@ function part11() {
   // we have to actually show the panel to get the bindings to instantiate
   notification.options.eventCallback = part12;
   // this scripts the plugin, triggering the popup notification
   try {
     gNewWindow.gBrowser.selectedBrowser.contentDocument.getElementById("test").wrappedJSObject.getObjectValue();
   } catch (e) {}
 }
 
-function part12() {
+function part12(type) {
+  if (type != "shown") {
+    return;
+  }
   let notification = PopupNotifications.getNotification("click-to-play-plugins", gNewWindow.gBrowser.selectedBrowser);
   notification.options.eventCallback = null;
   let centerAction = null;
   for (let action of notification.options.centerActions) {
     if (action.message == "Test") {
       centerAction = action;
       break;
     }

--- a/browser/base/content/test/browser_pluginnotification.js
+++ b/browser/base/content/test/browser_pluginnotification.js
@@ -713,16 +713,19 @@ function test18e() {
 function test18f() {
   var notification = PopupNotifications.getNotification("click-to-play-plugins", gTestBrowser);
   ok(notification, "Test 18f, Should have a click-to-play notification");
   ok(notification.dismissed, "Test 18f, notification should start dismissed");
   var plugin = gTestBrowser.contentDocument.getElementById("test");
   var objLoadingContent = plugin.QueryInterface(Ci.nsIObjectLoadingContent);
   ok(!objLoadingContent.activated, "Test 18f, Plugin should not be activated");
 
+  // XXXBAD: this code doesn't do what you think it does! it is actually
+  // observing the "removed" event of the old notification, since we create
+  // a *new* one when the plugin is clicked.
   notification.options.eventCallback = function() { executeSoon(test18g); };
   EventUtils.synthesizeMouseAtCenter(plugin, {}, gTestBrowser.contentWindow);
 }
 
 function test18g() {
   var notification = PopupNotifications.getNotification("click-to-play-plugins", gTestBrowser);
   ok(notification, "Test 18g, Should have a click-to-play notification");
   ok(!notification.dismissed, "Test 18g, notification should be open");
@@ -874,17 +877,20 @@ function test21a() {
     ok(!objLoadingContent.activated, "Test 21a, Plugin with id=" + plugin.id + " should not be activated");
   }
 
   // we have to actually show the panel to get the bindings to instantiate
   notification.options.eventCallback = test21b;
   notification.reshow();
 }
 
-function test21b() {
+function test21b(type) {
+  if (type != "shown") {
+    return;
+  }
   var notification = PopupNotifications.getNotification("click-to-play-plugins", gTestBrowser);
   notification.options.eventCallback = null;
   var centerAction = null;
   for (var action of notification.options.centerActions) {
     if (action.message == "Test") {
       centerAction = action;
       break;
     }
@@ -933,17 +939,20 @@ function test21c() {
     ok(!objLoadingContent.activated, "Test 21c, Plugin with id=" + plugin.id + " should not be activated");
   }
 
   // we have to actually show the panel to get the bindings to instantiate
   notification.options.eventCallback = test21d;
   notification.reshow();
 }
 
-function test21d() {
+function test21d(type) {
+  if (type != "shown") {
+    return;
+  }
   var notification = PopupNotifications.getNotification("click-to-play-plugins", gTestBrowser);
   notification.options.eventCallback = null;
 
   var centerAction = null;
   for (var action of notification.options.centerActions) {
     if (action.message == "Second Test") {
       centerAction = action;
       break;

--- a/browser/base/content/test/browser_plugins_added_dynamically.js
+++ b/browser/base/content/test/browser_plugins_added_dynamically.js
@@ -106,17 +106,20 @@ function testActivateAddSameTypePart2() 
   let popupNotification = PopupNotifications.getNotification("click-to-play-plugins", gTestBrowser);
   ok(popupNotification, "testActivateAddSameTypePart2: should have a click-to-play notification");
 
   // we have to actually show the panel to get the bindings to instantiate
   popupNotification.options.eventCallback = testActivateAddSameTypePart3;
   popupNotification.reshow();
 }
 
-function testActivateAddSameTypePart3() {
+function testActivateAddSameTypePart3(type) {
+  if (type != "shown") {
+    return;
+  }
   let popupNotification = PopupNotifications.getNotification("click-to-play-plugins", gTestBrowser);
   popupNotification.options.eventCallback = null;
   let centerAction = null;
   for (let action of popupNotification.options.centerActions) {
     if (action.message == "Test") {
       centerAction = action;
       break;
     }
@@ -184,17 +187,20 @@ function testActivateAddDifferentTypePar
   let popupNotification = PopupNotifications.getNotification("click-to-play-plugins", gTestBrowser);
   ok(popupNotification, "testActivateAddDifferentTypePart2: should have a click-to-play notification");
 
   // we have to actually show the panel to get the bindings to instantiate
   popupNotification.options.eventCallback = testActivateAddDifferentTypePart3;
   popupNotification.reshow();
 }
 
-function testActivateAddDifferentTypePart3() {
+function testActivateAddDifferentTypePart3(type) {
+  if (type != "shown") {
+    return;
+  }
   let popupNotification = PopupNotifications.getNotification("click-to-play-plugins", gTestBrowser);
   popupNotification.options.eventCallback = null;
   let centerAction = null;
   for (let action of popupNotification.options.centerActions) {
     if (action.message == "Test") {
       centerAction = action;
       break;
     }

--- a/browser/base/content/test/browser_popupNotification.js
+++ b/browser/base/content/test/browser_popupNotification.js
@@ -132,16 +132,19 @@ function basicNotification() {
     }
   ];
   this.options = {
     eventCallback: function (eventName) {
       switch (eventName) {
         case "dismissed":
           self.dismissalCallbackTriggered = true;
           break;
+        case "showing":
+          self.showingCallbackTriggered = true;
+          break;
         case "shown":
           self.shownCallbackTriggered = true;
           break;
         case "removed":
           self.removedCallbackTriggered = true;
           break;
       }
     }
@@ -853,32 +856,54 @@ var tests = [
          "notification2 anchor should be visible");
 
       dismissNotification(popup);
     },
     onHidden: function(popup) {
       this.notification1.remove();
       this.notification2.remove();
     }
+  },
+  { // Test #30 - Showing should be able to modify the popup data
+    run: function() {
+      this.notifyObj = new basicNotification();
+      var normalCallback = this.notifyObj.options.eventCallback;
+      this.notifyObj.options.eventCallback = function (eventName) {
+        if (eventName == "showing") {
+          this.mainAction.label = "Alternate Label";
+        }
+        normalCallback.call(this, eventName);
+      };
+      showNotification(this.notifyObj);
+    },
+    onShown: function(popup) {
+      // checkPopup checks for the matching label. Note that this assumes that
+      // this.notifyObj.mainAction is the same as notification.mainAction,
+      // which could be a problem if we ever decided to deep-copy.
+      checkPopup(popup, this.notifyObj);
+      triggerMainCommand(popup);
+    },
+    onHidden: function() { }
   }
 ];
 
 function showNotification(notifyObj) {
   return PopupNotifications.show(notifyObj.browser,
                                  notifyObj.id,
                                  notifyObj.message,
                                  notifyObj.anchorID,
                                  notifyObj.mainAction,
                                  notifyObj.secondaryActions,
                                  notifyObj.options);
 }
 
 function checkPopup(popup, notificationObj) {
   info("[Test #" + gTestIndex + "] checking popup");
 
+  ok(notificationObj.showingCallbackTriggered, "showing callback was triggered");
   ok(notificationObj.shownCallbackTriggered, "shown callback was triggered");
 
   let notifications = popup.childNodes;
   is(notifications.length, 1, "one notification displayed");
   let notification = notifications[0];
   if (!notification)
     return;
   let icon = document.getAnonymousElementByAttribute(notification, "class", "popup-notification-icon");

--- a/toolkit/modules/PopupNotifications.jsm
+++ b/toolkit/modules/PopupNotifications.jsm
@@ -5,16 +5,17 @@
 this.EXPORTED_SYMBOLS = ["PopupNotifications"];
 
 var Cc = Components.classes, Ci = Components.interfaces;
 
 Components.utils.import("resource://gre/modules/Services.jsm");
 
 const NOTIFICATION_EVENT_DISMISSED = "dismissed";
 const NOTIFICATION_EVENT_REMOVED = "removed";
+const NOTIFICATION_EVENT_SHOWING = "showing";
 const NOTIFICATION_EVENT_SHOWN = "shown";
 
 const ICON_SELECTOR = ".notification-anchor-icon";
 const ICON_ATTRIBUTE_SHOWING = "showing";
 
 const PREF_SECURITY_DELAY = "security.notification_enable_delay";
 
 let popupNotificationsMap = new WeakMap();
@@ -516,16 +517,19 @@ PopupNotifications.prototype = {
       // document rather than creating it ad hoc.
       popupnotification.hidden = false;
     }, this);
   },
 
   _showPanel: function PopupNotifications_showPanel(notificationsToShow, anchorElement) {
     this.panel.hidden = false;
 
+    notificationsToShow.forEach(function (n) {
+      this._fireCallback(n, NOTIFICATION_EVENT_SHOWING);
+    }, this);
     this._refreshPanel(notificationsToShow);
 
     if (this.isPanelOpen && this._currentAnchorElement == anchorElement)
       return;
 
     // If the panel is already open but we're changing anchors, we need to hide
     // it first.  Otherwise it can appear in the wrong spot.  (_hidePanel is
     // safe to call even if the panel is already hidden.)