diff --git a/toolkit/modules/SelectContentHelper.jsm b/toolkit/modules/SelectContentHelper.jsm
index ae2d0a3ca009..9be94182e827 100644
--- a/toolkit/modules/SelectContentHelper.jsm
+++ b/toolkit/modules/SelectContentHelper.jsm
@@ -14,6 +14,8 @@ XPCOMUtils.defineLazyModuleGetter(this, "BrowserUtils",
                                   "resource://gre/modules/BrowserUtils.jsm");
 XPCOMUtils.defineLazyServiceGetter(this, "DOMUtils",
                                    "@mozilla.org/inspector/dom-utils;1", "inIDOMUtils");
+XPCOMUtils.defineLazyModuleGetter(this, "DeferredTask",
+                                  "resource://gre/modules/DeferredTask.jsm");
 
 const kStateHover = 0x00000004; // NS_EVENT_STATE_HOVER
 
@@ -27,6 +29,7 @@ this.SelectContentHelper = function (aElement, aGlobal) {
   this.global = aGlobal;
   this.init();
   this.showDropDown();
+  this._updateTimer = new DeferredTask(this._update.bind(this), 0);
 }
 
 this.SelectContentHelper.prototype = {
@@ -36,7 +39,14 @@ this.SelectContentHelper.prototype = {
     this.global.addMessageListener("Forms:MouseOver", this);
     this.global.addMessageListener("Forms:MouseOut", this);
     this.global.addEventListener("pagehide", this);
-    this.global.addEventListener("mozhidedropdown", this);
+    let MutationObserver = this.element.ownerDocument.defaultView.MutationObserver;
+    this.mut = new MutationObserver(mutations => {
+      // Something changed the <select> while it was open, so
+      // we'll poke a DeferredTask to update the parent sometime
+      // in the very near future.
+      this._updateTimer.arm();
+    });
+    this.mut.observe(this.element, {childList: true, subtree: true});
   },
 
   uninit: function() {
@@ -45,9 +55,11 @@ this.SelectContentHelper.prototype = {
     this.global.removeMessageListener("Forms:MouseOver", this);
     this.global.removeMessageListener("Forms:MouseOut", this);
     this.global.removeEventListener("pagehide", this);
-    this.global.removeEventListener("mozhidedropdown", this);
     this.element = null;
     this.global = null;
+    this.mut.disconnect();
+    this._updateTimer.disarm();
+    this._updateTimer = null;
   },
 
   showDropDown: function() {
@@ -69,6 +81,15 @@ this.SelectContentHelper.prototype = {
     return buildOptionListForChildren(this.element);
   },
 
+  _update() {
+    // The <select> was updated while the dropdown was open.
+    // Let's send up a new list of options.
+    this.global.sendAsyncMessage("Forms:UpdateDropDown", {
+      options: this._buildOptionList(),
+      selectedIndex: this.element.selectedIndex,
+    });
+  },
+
   receiveMessage: function(message) {
     switch (message.name) {
       case "Forms:SelectDropDownItem":
diff --git a/toolkit/modules/SelectParentHelper.jsm b/toolkit/modules/SelectParentHelper.jsm
index 17691313f653..9ea58458c8c3 100644
--- a/toolkit/modules/SelectParentHelper.jsm
+++ b/toolkit/modules/SelectParentHelper.jsm
@@ -9,18 +9,22 @@ this.EXPORTED_SYMBOLS = [
 ];
 
 var currentBrowser = null;
+var currentMenulist = null;
+var currentZoom = 1;
 
 this.SelectParentHelper = {
   populate: function(menulist, items, selectedIndex, zoom) {
     // Clear the current contents of the popup
     menulist.menupopup.textContent = "";
+    currentZoom = zoom;
+    currentMenulist = menulist;
     populateChildren(menulist, items, selectedIndex, zoom);
   },
 
   open: function(browser, menulist, rect) {
     menulist.hidden = false;
     currentBrowser = browser;
-    this._registerListeners(menulist.menupopup);
+    this._registerListeners(browser, menulist.menupopup);
 
     menulist.menupopup.openPopupAtScreenRect("after_start", rect.left, rect.top, rect.width, rect.height, false, false);
     menulist.selectedItem.scrollIntoView();
@@ -52,26 +56,44 @@ this.SelectParentHelper = {
 
       case "popuphidden":
         currentBrowser.messageManager.sendAsyncMessage("Forms:DismissedDropDown", {});
-        currentBrowser = null;
         let popup = event.target;
-        this._unregisterListeners(popup);
+        this._unregisterListeners(currentBrowser, popup);
         popup.parentNode.hidden = true;
+        currentBrowser = null;
+        currentMenulist = null;
+        currentZoom = 1;
         break;
     }
   },
 
-  _registerListeners: function(popup) {
+  receiveMessage(msg) {
+    if (msg.name == "Forms:UpdateDropDown") {
+      // Sanity check - we'd better know what the currently
+      // opened menulist is, and what browser it belongs to...
+      if (!currentMenulist || !currentBrowser) {
+        return;
+      }
+
+      let options = msg.data.options;
+      let selectedIndex = msg.data.selectedIndex;
+      this.populate(currentMenulist, options, selectedIndex, currentZoom);
+    }
+  },
+
+  _registerListeners: function(browser, popup) {
     popup.addEventListener("command", this);
     popup.addEventListener("popuphidden", this);
     popup.addEventListener("mouseover", this);
     popup.addEventListener("mouseout", this);
+    browser.messageManager.addMessageListener("Forms:UpdateDropDown", this);
   },
 
-  _unregisterListeners: function(popup) {
+  _unregisterListeners: function(browser, popup) {
     popup.removeEventListener("command", this);
     popup.removeEventListener("popuphidden", this);
     popup.removeEventListener("mouseover", this);
     popup.removeEventListener("mouseout", this);
+    browser.messageManager.removeMessageListener("Forms:UpdateDropDown", this);
   },
 
 };