diff --git a/devtools/client/netmonitor/test/browser.ini b/devtools/client/netmonitor/test/browser.ini
index fcaa503c2a4b..9db665a92354 100644
--- a/devtools/client/netmonitor/test/browser.ini
+++ b/devtools/client/netmonitor/test/browser.ini
@@ -93,6 +93,7 @@ support-files =
 [browser_net_block-context.js]
 [browser_net_block-csp.js]
 [browser_net_block-draganddrop.js]
+[browser_net_block-extensions.js]
 [browser_net_block-pattern.js]
 skip-if = (os == 'linux') || debug && (os == 'win' || os == 'mac') # Bug 1603355
 [browser_net_cached-status.js]
diff --git a/devtools/client/netmonitor/test/browser_net_block-extensions.js b/devtools/client/netmonitor/test/browser_net_block-extensions.js
new file mode 100644
index 000000000000..9c4178056278
--- /dev/null
+++ b/devtools/client/netmonitor/test/browser_net_block-extensions.js
@@ -0,0 +1,99 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Test the requests that are blocked by extenstions show correctly.
+ */
+add_task(async function() {
+  const extensionName = "Test Blocker";
+  info(`Start loading the ${extensionName} extension...`);
+  const extension = ExtensionTestUtils.loadExtension({
+    manifest: {
+      name: extensionName,
+      permissions: ["*://example.com/", "webRequest", "webRequestBlocking"],
+    },
+    useAddonManager: "temporary",
+    background() {
+      // eslint-disable-next-line no-undef
+      browser.webRequest.onBeforeRequest.addListener(
+        () => {
+          return {
+            cancel: true,
+          };
+        },
+        {
+          urls: [
+            "http://example.com/browser/devtools/client/netmonitor/test/request_0",
+          ],
+        },
+        ["blocking"]
+      );
+      // eslint-disable-next-line no-undef
+      browser.test.sendMessage("ready");
+    },
+  });
+
+  await extension.startup();
+  await extension.awaitMessage("ready");
+
+  const { tab, monitor } = await initNetMonitor(SINGLE_GET_URL, {
+    requestCount: 2,
+  });
+
+  const { document } = monitor.panelWin;
+
+  info("Starting test... ");
+
+  const waitForNetworkEventsToComplete = waitForNetworkEvents(monitor, 2);
+  const waitForRequestsToRender = waitForDOM(
+    document,
+    ".requests-list-row-group"
+  );
+  const waitForReload = BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+
+  tab.linkedBrowser.reload();
+
+  await Promise.all([
+    waitForNetworkEventsToComplete,
+    waitForRequestsToRender,
+    waitForReload,
+  ]);
+
+  const requests = document.querySelectorAll(".request-list-item");
+
+  info("Assert the blocked request");
+  ok(
+    !!requests[1].querySelector(
+      ".requests-list-status .status-code-blocked-icon"
+    ),
+    "The request blocked status icon is visible"
+  );
+  is(
+    requests[1].querySelector(
+      ".requests-list-status .requests-list-status-code"
+    ).title,
+    "Blocked",
+    "The request status title is 'Blocked'"
+  );
+  is(
+    requests[1].querySelector(".requests-list-type").innerText,
+    "",
+    "The request shows no type"
+  );
+  is(
+    requests[1].querySelector(".requests-list-transferred").innerText,
+    `Blocked By ${extensionName}`,
+    "The request shows the blocking extension name"
+  );
+  is(
+    requests[1].querySelector(".requests-list-size").innerText,
+    "",
+    "The request shows no size"
+  );
+
+  await teardown(monitor);
+  info(`Unloading the ${extensionName} extension`);
+  await extension.unload();
+});