diff --git a/dom/chrome-webidl/ChannelWrapper.webidl b/dom/chrome-webidl/ChannelWrapper.webidl
index 6185f8ee48b1..9d6b07cd5dc6 100644
--- a/dom/chrome-webidl/ChannelWrapper.webidl
+++ b/dom/chrome-webidl/ChannelWrapper.webidl
@@ -95,9 +95,12 @@ interface ChannelWrapper : EventTarget {
 
   /**
    * Cancels the request with the given nsresult status code.
+   *
+   * The optional reason parameter should be one of the BLOCKING_REASON
+   * constants from nsILoadInfo.idl
    */
   [Throws]
-  void cancel(unsigned long result);
+  void cancel(unsigned long result, optional unsigned long reason = 0);
 
   /**
    * Redirects the wrapped HTTP channel to the given URI. For other channel
diff --git a/netwerk/base/nsILoadInfo.idl b/netwerk/base/nsILoadInfo.idl
index 4ffab7c3aa0c..88ba788a44a2 100644
--- a/netwerk/base/nsILoadInfo.idl
+++ b/netwerk/base/nsILoadInfo.idl
@@ -1188,6 +1188,8 @@ interface nsILoadInfo : nsISupports
   const uint32_t BLOCKING_REASON_CONTENT_POLICY_PRELOAD = 4006;
   // The reason used when SEC_REQUIRE_SAME_ORIGIN_* is set and not satisifed.
   const uint32_t BLOCKING_REASON_NOT_SAME_ORIGIN = 5000;
+  // The reason used when an extension cancels the request via the WebRequest api.
+  const uint32_t BLOCKING_REASON_EXTENSION_WEBREQUEST = 6000;
 
   /**
    * If the request associated with this load info was blocked by some of
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_cancelWithReason.js b/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_cancelWithReason.js
new file mode 100644
index 000000000000..7de301202a7f
--- /dev/null
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_cancelWithReason.js
@@ -0,0 +1,69 @@
+"use strict";
+
+const server = createHttpServer();
+const gServerUrl = `http://localhost:${server.identity.primaryPort}`;
+
+server.registerPathHandler("/dummy", (request, response) => {
+  response.setStatusLine(request.httpVersion, 200, "OK");
+  response.write("ok");
+});
+
+add_task(async function test_cancel_with_reason() {
+  let ext = ExtensionTestUtils.loadExtension({
+    manifest: {
+      applications: { gecko: { id: "cancel@test" } },
+      permissions: ["webRequest", "webRequestBlocking", "<all_urls>"],
+    },
+
+    background() {
+      browser.webRequest.onBeforeRequest.addListener(
+        () => {
+          return { cancel: true };
+        },
+        { urls: ["*://*/*"] },
+        ["blocking"]
+      );
+    },
+  });
+  await ext.startup();
+
+  let data = await new Promise(resolve => {
+    let ssm = Services.scriptSecurityManager;
+
+    let channel = NetUtil.newChannel({
+      uri: `${gServerUrl}/dummy`,
+      loadingPrincipal: ssm.createContentPrincipalFromOrigin(
+        "http://localhost"
+      ),
+      contentPolicyType: Ci.nsIContentPolicy.TYPE_XMLHTTPREQUEST,
+      securityFlags: Ci.nsILoadInfo.SEC_ALLOW_CROSS_ORIGIN_DATA_IS_NULL,
+    });
+
+    channel.asyncOpen({
+      QueryInterface: ChromeUtils.generateQI([Ci.nsIStreamListener]),
+
+      onStartRequest(request) {},
+
+      onStopRequest(request, statusCode) {
+        let properties = request.QueryInterface(Ci.nsIPropertyBag);
+        let id = properties.getProperty("cancelledByExtension");
+        let reason = request.loadInfo.requestBlockingReason;
+        resolve({ reason, id });
+      },
+
+      onDataAvailable() {},
+    });
+  });
+
+  Assert.equal(
+    Ci.nsILoadInfo.BLOCKING_REASON_EXTENSION_WEBREQUEST,
+    data.reason,
+    "extension cancelled request"
+  );
+  Assert.equal(
+    ext.id,
+    data.id,
+    "extension id attached to channel property bag"
+  );
+  await ext.unload();
+});
diff --git a/toolkit/components/extensions/test/xpcshell/xpcshell-common.ini b/toolkit/components/extensions/test/xpcshell/xpcshell-common.ini
index 2cddf2910c5f..e792b48e6daa 100644
--- a/toolkit/components/extensions/test/xpcshell/xpcshell-common.ini
+++ b/toolkit/components/extensions/test/xpcshell/xpcshell-common.ini
@@ -165,6 +165,7 @@ skip-if = true # Too frequent intermittent failures
 skip-if = os == "android" && debug
 [test_ext_webRequest_cached.js]
 skip-if = os == "android" && processor != "x86_64" # Bug 1573511
+[test_ext_webRequest_cancelWithReason.js]
 [test_ext_webRequest_filterResponseData.js]
 skip-if = os == "android" && debug
 [test_ext_webRequest_incognito.js]
diff --git a/toolkit/components/extensions/webrequest/ChannelWrapper.cpp b/toolkit/components/extensions/webrequest/ChannelWrapper.cpp
index 23c5381a51f6..90a9f7b331ee 100644
--- a/toolkit/components/extensions/webrequest/ChannelWrapper.cpp
+++ b/toolkit/components/extensions/webrequest/ChannelWrapper.cpp
@@ -206,9 +206,14 @@ void ChannelWrapper::ClearCachedAttributes() {
  * ...
  *****************************************************************************/
 
-void ChannelWrapper::Cancel(uint32_t aResult, ErrorResult& aRv) {
+void ChannelWrapper::Cancel(uint32_t aResult, uint32_t aReason,
+                            ErrorResult& aRv) {
   nsresult rv = NS_ERROR_UNEXPECTED;
   if (nsCOMPtr<nsIChannel> chan = MaybeChannel()) {
+    nsCOMPtr<nsILoadInfo> loadInfo = GetLoadInfo();
+    if (aReason > 0 && loadInfo) {
+      loadInfo->SetRequestBlockingReason(aReason);
+    }
     rv = chan->Cancel(nsresult(aResult));
     ErrorCheck();
   }
diff --git a/toolkit/components/extensions/webrequest/ChannelWrapper.h b/toolkit/components/extensions/webrequest/ChannelWrapper.h
index 83b0eed2985e..e07f1f20ebc1 100644
--- a/toolkit/components/extensions/webrequest/ChannelWrapper.h
+++ b/toolkit/components/extensions/webrequest/ChannelWrapper.h
@@ -135,7 +135,7 @@ class ChannelWrapper final : public DOMEventTargetHelper,
 
   void SetChannel(nsIChannel* aChannel);
 
-  void Cancel(uint32_t result, ErrorResult& aRv);
+  void Cancel(uint32_t result, uint32_t reason, ErrorResult& aRv);
 
   void RedirectTo(nsIURI* uri, ErrorResult& aRv);
   void UpgradeToSecure(ErrorResult& aRv);
diff --git a/toolkit/components/extensions/webrequest/WebRequest.jsm b/toolkit/components/extensions/webrequest/WebRequest.jsm
index 79cbf3430f70..bfd9d6a78999 100644
--- a/toolkit/components/extensions/webrequest/WebRequest.jsm
+++ b/toolkit/components/extensions/webrequest/WebRequest.jsm
@@ -972,7 +972,17 @@ HttpObserverManager = {
 
         if (result.cancel) {
           channel.suspended = false;
-          channel.cancel(Cr.NS_ERROR_ABORT);
+          channel.cancel(
+            Cr.NS_ERROR_ABORT,
+            Ci.nsILoadInfo.BLOCKING_REASON_EXTENSION_WEBREQUEST
+          );
+          let { policy } = opts;
+          if (policy) {
+            let properties = channel.channel.QueryInterface(
+              Ci.nsIWritablePropertyBag
+            );
+            properties.setProperty("cancelledByExtension", policy.id);
+          }
           return;
         }