--- a/build/pgo/server-locations.txt
+++ b/build/pgo/server-locations.txt
@@ -290,8 +290,11 @@ https://tls1.example.com:443        priv
 https://mochitest.youtube.com:443
 
 # Hosts for stylo blocklist tests
 http://stylo-blocklist.com:80          privileged
 http://test.stylo-blocklist.com:80     privileged
 
 # Host for U2F localhost tests
 https://localhost:443
+
+# Host for Web Authentication alternate-port tests
+https://webauthn.example.com:8443

--- a/dom/webauthn/tests/browser/browser.ini
+++ b/dom/webauthn/tests/browser/browser.ini
@@ -3,9 +3,10 @@ support-files =
   tab_webauthn_result.html
   tab_webauthn_success.html
   ../cbor/*
   ../pkijs/*
   ../u2futil.js
 skip-if = !e10s
 
 [browser_abort_visibility.js]
+[browser_alternate_ports.js]
 [browser_webauthn_telemetry.js]

new file mode 100644
--- /dev/null
+++ b/dom/webauthn/tests/browser/browser_alternate_ports.js
@@ -0,0 +1,71 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const TEST_URL_8443 = "https://webauthn.example.com:8443/browser/dom/webauthn/tests/browser/tab_webauthn_result.html";
+
+async function assertStatus(tab, expected) {
+  let actual = await ContentTask.spawn(tab.linkedBrowser, null, async function () {
+    return content.document.getElementById("status").value;
+  });
+  is(actual, expected, "webauthn request " + expected);
+}
+
+async function waitForStatus(tab, expected) {
+  await ContentTask.spawn(tab.linkedBrowser, [expected], async function (expected) {
+    return ContentTaskUtils.waitForCondition(() => {
+      return content.document.getElementById("status").value == expected;
+    });
+  });
+
+  await assertStatus(tab, expected);
+}
+
+function startMakeCredentialRequest(tab) {
+  return ContentTask.spawn(tab.linkedBrowser, null, async function () {
+    const cose_alg_ECDSA_w_SHA256 = -7;
+
+    let publicKey = {
+      rp: {id: "example.com", name: "none", icon: "none"},
+      user: {id: new Uint8Array(), name: "none", icon: "none", displayName: "none"},
+      challenge: content.crypto.getRandomValues(new Uint8Array(16)),
+      timeout: 5000, // the minimum timeout is actually 15 seconds
+      pubKeyCredParams: [{type: "public-key", alg: cose_alg_ECDSA_w_SHA256}],
+    };
+
+    let status = content.document.getElementById("status");
+
+    content.navigator.credentials.create({publicKey}).then(() => {
+      status.value = "completed";
+    }).catch(() => {
+      status.value = "aborted";
+    });
+
+    status.value = "pending";
+  });
+}
+
+// Test that MakeCredential() and GetAssertion() requests
+// on alternate ports still function with the same RP ID
+add_task(async function test_alternate_port() {
+  Services.prefs.setBoolPref("security.webauth.webauthn", true);
+  Services.prefs.setBoolPref("security.webauth.webauthn_enable_softtoken", true);
+  Services.prefs.setBoolPref("security.webauth.webauthn_enable_usbtoken", false);
+
+  // Create a new tab for the MakeCredential() request.
+  let tab_8443 = await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_URL_8443);
+
+  // Run the Make Credential request.
+  await startMakeCredentialRequest(tab_8443);
+  await assertStatus(tab_8443, "completed");
+
+  // Close tabs.
+  await BrowserTestUtils.removeTab(tab_8443);
+
+  // Cleanup.
+  Services.prefs.clearUserPref("security.webauth.webauthn");
+  Services.prefs.clearUserPref("security.webauth.webauthn_enable_softtoken");
+  Services.prefs.clearUserPref("security.webauth.webauthn_enable_usbtoken");
+});