--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -23,16 +23,17 @@
 #include "mozilla/dom/workers/ServiceWorkerManager.h"
 #include "mozilla/EventStateManager.h"
 #include "mozilla/LoadInfo.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/Services.h"
 #include "mozilla/StartupTimeline.h"
 #include "mozilla/Telemetry.h"
 #include "mozilla/unused.h"
+#include "Navigator.h"
 #include "URIUtils.h"
 
 #include "nsIContent.h"
 #include "nsIContentInlines.h"
 #include "nsIDocument.h"
 #include "nsIDOMDocument.h"
 #include "nsIDOMElement.h"
 
@@ -3123,16 +3124,48 @@ NS_IMETHODIMP
 nsDocShell::NameEquals(const char16_t* aName, bool* aResult)
 {
   NS_ENSURE_ARG_POINTER(aName);
   NS_ENSURE_ARG_POINTER(aResult);
   *aResult = mName.Equals(aName);
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDocShell::GetCustomUserAgent(nsAString& aCustomUserAgent)
+{
+  aCustomUserAgent = mCustomUserAgent;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsDocShell::SetCustomUserAgent(const nsAString& aCustomUserAgent)
+{
+  mCustomUserAgent = aCustomUserAgent;
+  RefPtr<nsGlobalWindow> win = mScriptGlobal ?
+    mScriptGlobal->GetCurrentInnerWindowInternal() : nullptr;
+  if (win) {
+    ErrorResult ignored;
+    Navigator* navigator = win->GetNavigator(ignored);
+    ignored.SuppressException();
+    if (navigator) {
+      navigator->ClearUserAgentCache();
+    }
+  }
+
+  uint32_t childCount = mChildList.Length();
+  for (uint32_t i = 0; i < childCount; ++i) {
+    nsCOMPtr<nsIDocShell> childShell = do_QueryInterface(ChildAt(i));
+    if (childShell) {
+      childShell->SetCustomUserAgent(aCustomUserAgent);
+    }
+  }
+  return NS_OK;
+}
+
 /* virtual */ int32_t
 nsDocShell::ItemType()
 {
   return mItemType;
 }
 
 NS_IMETHODIMP
 nsDocShell::GetItemType(int32_t* aItemType)
@@ -3250,16 +3283,17 @@ nsDocShell::SetDocLoaderParent(nsDocLoad
   }
 
   // Curse ambiguous nsISupports inheritance!
   nsISupports* parent = GetAsSupports(aParent);
 
   // If parent is another docshell, we inherit all their flags for
   // allowing plugins, scripting etc.
   bool value;
+  nsString customUserAgent;
   nsCOMPtr<nsIDocShell> parentAsDocShell(do_QueryInterface(parent));
   if (parentAsDocShell) {
     if (mAllowPlugins && NS_SUCCEEDED(parentAsDocShell->GetAllowPlugins(&value))) {
       SetAllowPlugins(value);
     }
     if (mAllowJavascript && NS_SUCCEEDED(parentAsDocShell->GetAllowJavascript(&value))) {
       SetAllowJavascript(value);
     }
@@ -3279,16 +3313,20 @@ nsDocShell::SetDocLoaderParent(nsDocLoad
     SetAllowContentRetargeting(mAllowContentRetargeting &&
       parentAsDocShell->GetAllowContentRetargetingOnChildren());
     if (NS_SUCCEEDED(parentAsDocShell->GetIsActive(&value))) {
       SetIsActive(value);
     }
     if (parentAsDocShell->GetIsPrerendered()) {
       SetIsPrerendered(true);
     }
+    if (NS_SUCCEEDED(parentAsDocShell->GetCustomUserAgent(customUserAgent)) &&
+        !customUserAgent.IsEmpty()) {
+      SetCustomUserAgent(customUserAgent);
+    }
     if (NS_FAILED(parentAsDocShell->GetAllowDNSPrefetch(&value))) {
       value = false;
     }
     SetAllowDNSPrefetch(mAllowDNSPrefetch && value);
     value = parentAsDocShell->GetAffectPrivateSessionLifetime();
     SetAffectPrivateSessionLifetime(value);
     uint32_t flags;
     if (NS_SUCCEEDED(parentAsDocShell->GetDefaultLoadFlags(&flags))) {

--- a/docshell/base/nsDocShell.h
+++ b/docshell/base/nsDocShell.h
@@ -788,16 +788,17 @@ protected:
   FrameType GetInheritedFrameType();
 
   bool HasUnloadedParent();
 
   // Dimensions of the docshell
   nsIntRect mBounds;
   nsString mName;
   nsString mTitle;
+  nsString mCustomUserAgent;
 
   /**
    * Content-Type Hint of the most-recently initiated load. Used for
    * session history entries.
    */
   nsCString mContentTypeHint;
   nsIntPoint mDefaultScrollbarPref; // persistent across doc loads
 

--- a/docshell/base/nsIDocShell.idl
+++ b/docshell/base/nsIDocShell.idl
@@ -38,17 +38,17 @@ interface nsIPrincipal;
 interface nsIWebBrowserPrint;
 interface nsIPrivacyTransitionObserver;
 interface nsIReflowObserver;
 interface nsIScrollObserver;
 interface nsITabParent;
 
 typedef unsigned long nsLoadFlags;
 
-[scriptable, builtinclass, uuid(63adb599-6dc9-4746-972e-c22e9018020b)]
+[scriptable, builtinclass, uuid(bc3524bd-023c-4fc8-ace1-472bc999fb12)]
 interface nsIDocShell : nsIDocShellTreeItem
 {
   /**
    * Loads a given URI.  This will give priority to loading the requested URI
    * in the object implementing	this interface.  If it can't be loaded here
    * however, the URL dispatcher will go through its normal process of content
    * loading.
    *
@@ -233,29 +233,34 @@ interface nsIDocShell : nsIDocShellTreeI
 
   /**
    * This attribute allows chrome to tie in to handle DOM events that may
    * be of interest to chrome.
    */
   attribute nsIDOMEventTarget chromeEventHandler;
 
   /**
+    * This allows chrome to set a custom User agent on a specific docshell
+    */
+  attribute DOMString customUserAgent;
+
+  /**
    * Whether to allow plugin execution
    */
   attribute boolean allowPlugins;
 
   /**
    * Whether to allow Javascript execution
    */
   attribute boolean allowJavascript;
 
   /**
    * Attribute stating if refresh based redirects can be allowed
    */
-  attribute  boolean allowMetaRedirects;
+  attribute boolean allowMetaRedirects;
 
   /**
    * Attribute stating if it should allow subframes (framesets/iframes) or not
    */
   attribute boolean allowSubframes;
 
   /**
    * Attribute stating whether or not images should be loaded.

--- a/docshell/test/browser/browser.ini
+++ b/docshell/test/browser/browser.ini
@@ -79,13 +79,14 @@ skip-if = e10s # Bug 1220927 - Test trie
 [browser_bug852909.js]
 [browser_bug92473.js]
 [browser_uriFixupIntegration.js]
 [browser_loadDisallowInherit.js]
 [browser_loadURI.js]
 [browser_multiple_pushState.js]
 [browser_onbeforeunload_navigation.js]
 [browser_search_notification.js]
+[browser_ua_emulation.js]
 [browser_timelineMarkers-01.js]
 [browser_timelineMarkers-02.js]
 [browser_timelineMarkers-03.js]
 [browser_timelineMarkers-04.js]
 [browser_timelineMarkers-05.js]

new file mode 100644
--- /dev/null
+++ b/docshell/test/browser/browser_ua_emulation.js
@@ -0,0 +1,52 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the docShell UA emulation works
+add_task(function*() {
+  yield openUrl("data:text/html;charset=utf-8,<iframe id='test-iframe'></iframe>");
+
+  let docshell = content.QueryInterface(Ci.nsIInterfaceRequestor)
+                        .getInterface(Ci.nsIWebNavigation)
+                        .QueryInterface(Ci.nsIDocShell);
+  is(docshell.customUserAgent, "", "There should initially be no customUserAgent");
+
+  docshell.customUserAgent = "foo";
+  is(content.navigator.userAgent, "foo", "The user agent should be changed to foo");
+
+  let frameWin = content.document.querySelector("#test-iframe").contentWindow;
+  is(frameWin.navigator.userAgent, "foo", "The UA should be passed on to frames.");
+
+  let newFrame = content.document.createElement("iframe");
+  content.document.body.appendChild(newFrame);
+
+  let newFrameWin = newFrame.contentWindow;
+  is(newFrameWin.navigator.userAgent, "foo", "Newly created frames should use the new UA");
+
+  newFrameWin.location.reload();
+  yield waitForEvent(newFrameWin, "load");
+
+  is(newFrameWin.navigator.userAgent, "foo", "New UA should persist across reloads");
+  gBrowser.removeCurrentTab();
+});
+
+function waitForEvent(target, event) {
+  return new Promise(function(resolve) {
+    target.addEventListener(event, resolve);
+  });
+}
+
+function openUrl(url) {
+  return new Promise(function(resolve, reject) {
+    window.focus();
+
+    let tab = window.gBrowser.selectedTab = window.gBrowser.addTab(url);
+    let linkedBrowser = tab.linkedBrowser;
+
+    linkedBrowser.addEventListener("load", function onload() {
+      linkedBrowser.removeEventListener("load", onload, true);
+      resolve(tab);
+    }, true);
+  });
+}

--- a/dom/base/Navigator.cpp
+++ b/dom/base/Navigator.cpp
@@ -367,21 +367,32 @@ Navigator::Invalidate()
 //*****************************************************************************
 
 NS_IMETHODIMP
 Navigator::GetUserAgent(nsAString& aUserAgent)
 {
   nsCOMPtr<nsIURI> codebaseURI;
   nsCOMPtr<nsPIDOMWindow> window;
 
-  if (mWindow && mWindow->GetDocShell()) {
+  if (mWindow) {
     window = mWindow;
-    nsIDocument* doc = mWindow->GetExtantDoc();
-    if (doc) {
-      doc->NodePrincipal()->GetURI(getter_AddRefs(codebaseURI));
+    nsIDocShell* docshell = window->GetDocShell();
+    nsString customUserAgent;
+    if (docshell) {
+      docshell->GetCustomUserAgent(customUserAgent);
+
+      if (!customUserAgent.IsEmpty()) {
+        aUserAgent = customUserAgent;
+        return NS_OK;
+      }
+
+      nsIDocument* doc = mWindow->GetExtantDoc();
+      if (doc) {
+        doc->NodePrincipal()->GetURI(getter_AddRefs(codebaseURI));
+      }
     }
   }
 
   return GetUserAgent(window, codebaseURI, nsContentUtils::IsCallerChrome(),
                       aUserAgent);
 }
 
 NS_IMETHODIMP
@@ -2736,16 +2747,22 @@ Navigator::AppName(nsAString& aAppName, 
       aAppName = override;
       return;
     }
   }
 
   aAppName.AssignLiteral("Netscape");
 }
 
+void
+Navigator::ClearUserAgentCache()
+{
+  NavigatorBinding::ClearCachedUserAgentValue(this);
+}
+
 nsresult
 Navigator::GetUserAgent(nsPIDOMWindow* aWindow, nsIURI* aURI,
                         bool aIsCallerChrome,
                         nsAString& aUserAgent)
 {
   MOZ_ASSERT(NS_IsMainThread());
 
   if (!aIsCallerChrome) {

--- a/dom/base/Navigator.h
+++ b/dom/base/Navigator.h
@@ -177,16 +177,20 @@ public:
   static nsresult GetAppVersion(nsAString& aAppVersion,
                                 bool aUsePrefOverriddenValue);
 
   static nsresult GetUserAgent(nsPIDOMWindow* aWindow,
                                nsIURI* aURI,
                                bool aIsCallerChrome,
                                nsAString& aUserAgent);
 
+  // Clears the user agent cache by calling:
+  // NavigatorBinding::ClearCachedUserAgentValue(this);
+  void ClearUserAgentCache();
+
   already_AddRefed<Promise> GetDataStores(const nsAString& aName,
                                           const nsAString& aOwner,
                                           ErrorResult& aRv);
 
   // Feature Detection API
   already_AddRefed<Promise> GetFeature(const nsAString& aName,
                                        ErrorResult& aRv);
 

--- a/dom/webidl/Navigator.webidl
+++ b/dom/webidl/Navigator.webidl
@@ -36,17 +36,17 @@ interface NavigatorID {
   [Constant, Cached]
   readonly attribute DOMString appCodeName; // constant "Mozilla"
   [Constant, Cached]
   readonly attribute DOMString appName;
   [Constant, Cached]
   readonly attribute DOMString appVersion;
   [Constant, Cached]
   readonly attribute DOMString platform;
-  [Constant, Cached, Throws=Workers]
+  [Pure, Cached, Throws=Workers]
   readonly attribute DOMString userAgent;
   [Constant, Cached]
   readonly attribute DOMString product; // constant "Gecko"
 
   // Everyone but WebKit/Blink supports this.  See bug 679971.
   [Exposed=Window]
   boolean taintEnabled(); // constant false
 };