--- a/remote/test/browser/browser.ini
+++ b/remote/test/browser/browser.ini
@@ -1,12 +1,13 @@
 [DEFAULT]
 tags = remote
 subsuite = remote
 prefs = remote.enabled=true
 support-files =
   chrome-remote-interface.js
   head.js
 
+[browser_agent.js]
 [browser_cdp.js]
 [browser_main_target.js]
 [browser_session.js]
 [browser_tabs.js]

new file mode 100644
--- /dev/null
+++ b/remote/test/browser/browser_agent.js
@@ -0,0 +1,109 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { Preferences } = ChromeUtils.import(
+  "resource://gre/modules/Preferences.jsm"
+);
+
+const URL = Services.io.newURI("http://localhost:0");
+
+// set up test conditions and clean up
+function add_agent_task(taskFn) {
+  add_plain_task(async () => {
+    try {
+      await RemoteAgent.close();
+      await taskFn();
+    } finally {
+      Preferences.reset("remote.enabled");
+      Preferences.reset("remote.force-local");
+    }
+  });
+}
+
+add_agent_task(async function listening() {
+  is(RemoteAgent.listening, false, "Agent is not listening");
+  await RemoteAgent.listen(URL);
+  is(RemoteAgent.listening, true, "Agent is listening");
+});
+
+add_agent_task(async function listen() {
+  const port = getNonAtomicFreePort();
+
+  let boundURL;
+  function observer(subject, topic, data) {
+    Services.obs.removeObserver(observer, topic);
+    boundURL = Services.io.newURI(data);
+  }
+  Services.obs.addObserver(observer, "remote-listening");
+
+  await RemoteAgent.listen("http://localhost:" + port);
+  isnot(boundURL, undefined, "remote-listening observer not notified");
+  is(
+    boundURL.port,
+    port,
+    `expected default port ${port}, but got ${boundURL.port}`
+  );
+});
+
+add_agent_task(async function listenWhenDisabled() {
+  Preferences.set("remote.enabled", false);
+  try {
+    await RemoteAgent.listen(URL);
+    fail("listen() did not return exception");
+  } catch (e) {
+    is(e.result, Cr.NS_ERROR_NOT_AVAILABLE);
+    is(e.message, "Disabled by preference");
+  }
+});
+
+// TODO(ato): https://bugzil.la/1590829
+add_agent_task(async function listenTakesString() {
+  await RemoteAgent.listen("http://localhost:0");
+  await RemoteAgent.close();
+});
+
+// TODO(ato): https://bugzil.la/1590829
+add_agent_task(async function listenNonURL() {
+  try {
+    await RemoteAgent.listen("foobar");
+    fail("listen() did not reject non-URL");
+  } catch (e) {
+    is(e.result, Cr.NS_ERROR_MALFORMED_URI);
+  }
+});
+
+add_agent_task(async function listenRestrictedToLoopbackDevice() {
+  try {
+    await RemoteAgent.listen("http://0.0.0.0:0");
+    fail("listen() did not reject non-loopback device");
+  } catch (e) {
+    is(e.result, Cr.NS_ERROR_ILLEGAL_VALUE);
+    is(e.message, "Restricted to loopback devices");
+  }
+});
+
+add_agent_task(async function listenNonLoopbackDevice() {
+  Preferences.set("remote.force-local", false);
+  await RemoteAgent.listen("http://0.0.0.0:0");
+});
+
+add_agent_task(async function test_close() {
+  await RemoteAgent.listen(URL);
+  await RemoteAgent.close();
+  // no-op when not listening
+  await RemoteAgent.close();
+});
+
+function getNonAtomicFreePort() {
+  const so = Cc["@mozilla.org/network/server-socket;1"].createInstance(
+    Ci.nsIServerSocket
+  );
+  try {
+    so.init(-1, true /* aLoopbackOnly */, -1 /* aBackLog */);
+    return so.port;
+  } finally {
+    so.close();
+  }
+}