diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 3e604f9cde39..38d82d2fe32c 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -301,6 +301,7 @@ bool nsContentUtils::sAnimationsAPIElementAnimateEnabled = false;
 bool nsContentUtils::sGetBoxQuadsEnabled = false;
 bool nsContentUtils::sSkipCursorMoveForSameValueSet = false;
 bool nsContentUtils::sRequestIdleCallbackEnabled = false;
+bool nsContentUtils::sLowerNetworkPriority = false;
 
 int32_t nsContentUtils::sPrivacyMaxInnerWidth = 1000;
 int32_t nsContentUtils::sPrivacyMaxInnerHeight = 1000;
@@ -715,6 +716,9 @@ nsContentUtils::Init()
   Preferences::AddBoolVarCache(&sRequestIdleCallbackEnabled,
                                "dom.requestIdleCallback.enabled", false);
 
+  Preferences::AddBoolVarCache(&sLowerNetworkPriority,
+                               "privacy.trackingprotection.lower_network_priority", false);
+
   Element::InitCCCallbacks();
 
   Unused << nsRFPService::GetOrCreate();
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 6852ef009aa1..41fedbf90aee 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2953,6 +2953,10 @@ public:
   static bool
   GetUserIsInteracting();
 
+  // Check pref "privacy.trackingprotection.lower_network_priority" to see
+  // if we want to lower the priority of the channel.
+  static bool IsLowerNetworkPriority() { return sLowerNetworkPriority; }
+
 private:
   static bool InitializeEventTable();
 
@@ -3079,6 +3083,7 @@ private:
   static bool sGetBoxQuadsEnabled;
   static bool sSkipCursorMoveForSameValueSet;
   static bool sRequestIdleCallbackEnabled;
+  static bool sLowerNetworkPriority;
   static uint32_t sCookiesLifetimePolicy;
   static uint32_t sCookiesBehavior;
 
diff --git a/dom/xhr/XMLHttpRequestMainThread.cpp b/dom/xhr/XMLHttpRequestMainThread.cpp
index f8e589fec1b6..2774a056ee9c 100644
--- a/dom/xhr/XMLHttpRequestMainThread.cpp
+++ b/dom/xhr/XMLHttpRequestMainThread.cpp
@@ -46,6 +46,7 @@
 #include "nsIAuthPrompt2.h"
 #include "nsIOutputStream.h"
 #include "nsISupportsPrimitives.h"
+#include "nsISupportsPriority.h"
 #include "nsIInterfaceRequestorUtils.h"
 #include "nsStreamUtils.h"
 #include "nsThreadUtils.h"
@@ -2550,6 +2551,37 @@ XMLHttpRequestMainThread::CreateChannel()
   return NS_OK;
 }
 
+void
+XMLHttpRequestMainThread::MaybeLowerChannelPriority()
+{
+  nsCOMPtr<nsIDocument> doc = GetDocumentIfCurrent();
+  if (!doc) {
+    return;
+  }
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(GetOwnerGlobal())) {
+    return;
+  }
+
+  JSContext* cx = jsapi.cx();
+  nsAutoCString fileNameString;
+  if (!nsJSUtils::GetCallingLocation(cx, fileNameString)) {
+    return;
+  }
+
+  if (!doc->IsScriptTracking(fileNameString)) {
+    return;
+  }
+
+  nsCOMPtr<nsISupportsPriority> p = do_QueryInterface(mChannel);
+  if (!p) {
+    return;
+  }
+
+  p->SetPriority(nsISupportsPriority::PRIORITY_LOWEST);
+}
+
 nsresult
 XMLHttpRequestMainThread::InitiateFetch(nsIInputStream* aUploadStream,
                                         int64_t aUploadLength,
@@ -2738,6 +2770,12 @@ XMLHttpRequestMainThread::InitiateFetch(nsIInputStream* aUploadStream,
   // Make sure to hold a strong reference so that we don't leak the wrapper.
   nsCOMPtr<nsIStreamListener> listener = new net::nsStreamListenerWrapper(this);
 
+  // Check if this XHR is created from a tracking script.
+  // If yes, lower the channel's priority.
+  if (nsContentUtils::IsLowerNetworkPriority()) {
+    MaybeLowerChannelPriority();
+  }
+
   // Start reading from the channel
   rv = mChannel->AsyncOpen2(listener);
   listener = nullptr;
diff --git a/dom/xhr/XMLHttpRequestMainThread.h b/dom/xhr/XMLHttpRequestMainThread.h
index 637aa164c1b1..e15a8d847d92 100644
--- a/dom/xhr/XMLHttpRequestMainThread.h
+++ b/dom/xhr/XMLHttpRequestMainThread.h
@@ -314,6 +314,8 @@ private:
   // response header is supported.
   static bool IsLowercaseResponseHeader();
 
+  void MaybeLowerChannelPriority();
+
 public:
   virtual void
   Send(JSContext* /*aCx*/, ErrorResult& aRv) override