--- a/browser/extensions/followonsearch/content/followonsearch-fs.js
+++ b/browser/extensions/followonsearch/content/followonsearch-fs.js
@@ -87,16 +87,25 @@ let searchDomains = [{
     "www.google.com.vn", "www.google.vu", "www.google.ws", "www.google.co.za",
     "www.google.co.zm", "www.google.co.zw",
   ],
   "search": "q",
   "prefix": ["client"],
   "followOnSearch": "oq",
   "codes": ["firefox-b-ab", "firefox-b", "firefox-b-1-ab", "firefox-b-1"],
   "sap": "google",
+}, {
+  // This is intended only for tests.
+  "domains": [ "mochi.test" ],
+  "search": "m",
+  "prefix": ["mt"],
+  "followOnSearch": "mtfo",
+  "reportPrefix": "form",
+  "codes": ["TEST"],
+  "sap": "mochitest"
 }];
 
 function getSearchDomainCodes(host) {
   for (let domainInfo of searchDomains) {
     if (domainInfo.domains.includes(host)) {
       return domainInfo;
     }
   }

--- a/browser/extensions/followonsearch/moz.build
+++ b/browser/extensions/followonsearch/moz.build
@@ -7,9 +7,11 @@
 with Files("**"):
     BUG_COMPONENT = ("Firefox", "Search")
 
 FINAL_TARGET_FILES.features['followonsearch@mozilla.com'] += [
   'bootstrap.js',
   'install.rdf',
 ]
 
+BROWSER_CHROME_MANIFESTS += ['test/browser/browser.ini']
+
 JAR_MANIFESTS += ['jar.mn']

new file mode 100644
--- /dev/null
+++ b/browser/extensions/followonsearch/test/browser/.eslintrc.js
@@ -0,0 +1,7 @@
+"use strict";
+
+module.exports = {
+  "extends": [
+    "plugin:mozilla/browser-test",
+  ],
+};

new file mode 100644
--- /dev/null
+++ b/browser/extensions/followonsearch/test/browser/browser.ini
@@ -0,0 +1,7 @@
+[DEFAULT]
+
+[browser_followOnTelemetry.js]
+support-files =
+  test.html
+  test2.html
+  testEngine.xml

new file mode 100644
--- /dev/null
+++ b/browser/extensions/followonsearch/test/browser/browser_followOnTelemetry.js
@@ -0,0 +1,66 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+ChromeUtils.defineModuleGetter(this, "SearchTestUtils",
+  "resource://testing-common/SearchTestUtils.jsm");
+
+SearchTestUtils.init(Assert, registerCleanupFunction);
+
+const BASE_URL = "http://mochi.test:8888/browser/browser/extensions/followonsearch/test/browser/";
+const TEST_ENGINE_BASENAME = "testEngine.xml";
+
+add_task(async function test_followOnSearchTelemetry() {
+  let engine = await SearchTestUtils.promiseNewSearchEngine(
+    getRootDirectory(gTestPath) + TEST_ENGINE_BASENAME,
+    registerCleanupFunction);
+  let oldCurrentEngine = Services.search.currentEngine;
+  Services.search.currentEngine = engine;
+
+  let histogram = Services.telemetry.getKeyedHistogramById("SEARCH_COUNTS");
+  histogram.clear();
+
+  registerCleanupFunction(() => Services.search.currentEngine = oldCurrentEngine);
+
+  await BrowserTestUtils.withNewTab({gBrowser}, async browser => {
+    // Open the initial search page via entering a search on the URL bar.
+    let loadPromise = BrowserTestUtils.waitForLocationChange(gBrowser,
+      `${BASE_URL}test.html?searchm=test&mt=TEST`);
+
+    gURLBar.focus();
+    EventUtils.sendString("test");
+    EventUtils.sendKey("return");
+
+    await loadPromise;
+
+    // Perform a follow-on search, selecting the form in the page.
+    loadPromise = BrowserTestUtils.waitForLocationChange(gBrowser,
+      `${BASE_URL}test2.html?mtfo=followonsearchtest&mt=TEST&m=test`);
+
+    await ContentTask.spawn(browser, null, function() {
+      content.document.getElementById("submit").click();
+    });
+
+    await loadPromise;
+
+    let snapshot;
+
+    // We have to wait for the snapshot to come in, as there's async functionality
+    // in the extension.
+    await TestUtils.waitForCondition(() => {
+      snapshot = histogram.snapshot();
+      return "mochitest.follow-on:unknown:TEST" in snapshot;
+    });
+    Assert.ok("mochitest.follow-on:unknown:TEST" in snapshot,
+      "Histogram should have an entry for the follow-on search.");
+    Assert.deepEqual(snapshot["mochitest.follow-on:unknown:TEST"], {
+      counts: [ 1, 0, 0 ],
+      histogram_type: 4,
+      max: 2,
+      min: 1,
+      ranges: [ 0, 1, 2 ],
+      sum: 1,
+    }, "Histogram should have the correct snapshot data");
+  });
+});

new file mode 100644
--- /dev/null
+++ b/browser/extensions/followonsearch/test/browser/test.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8" />
+  <title>Follow-on Search Test</title>
+</head>
+<body>
+  <form method="get" action="test2.html">
+    <input type="text" name="mtfo" value="followonsearchtest"/>
+    <input type="text" name="mt" value="TEST"/>
+    <input type="text" name="m" value="test"/>
+    <input type="submit" id="submit" value="submit"/>
+  </form>
+</body>
+</html>

new file mode 100644
--- /dev/null
+++ b/browser/extensions/followonsearch/test/browser/test2.html
@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8" />
+  <title>Follow-on Search Test Final Page</title>
+</head>
+<body></body>
+</html>

new file mode 100644
--- /dev/null
+++ b/browser/extensions/followonsearch/test/browser/testEngine.xml
@@ -0,0 +1,15 @@
+<!-- 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/. -->
+
+<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/"
+                       xmlns:moz="http://www.mozilla.org/2006/browser/search/">
+  <ShortName>Mochitest</ShortName>
+  <Description>Mochitest Engine</Description>
+  <InputEncoding>UTF-8</InputEncoding>
+  <Image width="16" height="16">data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAIAAACQkWg2AAABGklEQVQoz2NgGB6AnZ1dUlJSXl4eSDIyMhLW4Ovr%2B%2Fr168uXL69Zs4YoG%2BLi4i5dusTExMTGxsbNzd3f37937976%2BnpmZmagbHR09J49e5YvX66kpATVEBYW9ubNm2nTphkbG7e2tp44cQLIuHfvXm5urpaWFlDKysqqu7v73LlzECMYIiIiHj58mJCQoKKicvXq1bS0NKBgW1vbjh074uPjgeqAXE1NzSdPnvDz84M0AEUvXLgAsW379u1z5swBen3jxo2zZ892cHB4%2BvQp0KlAfwI1cHJyghQFBwfv2rULokFXV%2FfixYu7d%2B8GGqGgoMDKyrpu3br9%2B%2FcDuXl5eVA%2FAEWBfoWHAdAYoNuAYQ0XAeoUERFhGDYAAPoUaT2dfWJuAAAAAElFTkSuQmCC</Image>
+  <Url type="text/html" method="GET" template="http://mochi.test:8888/browser/browser/extensions/followonsearch/test/browser/test.html?search" rel="searchform">
+    <Param name="m" value="{searchTerms}"/>
+    <Param name="mt" value="TEST"/>
+  </Url>
+</OpenSearchDescription>