diff --git a/docshell/test/unit/data/engine.xml b/docshell/test/unit/data/engine.xml
new file mode 100644
index 000000000000..3a2bd85c1b09
--- /dev/null
+++ b/docshell/test/unit/data/engine.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<SearchPlugin xmlns="http://www.mozilla.org/2006/browser/search/">
+<ShortName>test_urifixup_search_engine</ShortName>
+<Description>test_urifixup_search_engine</Description>
+<InputEncoding>UTF-8</InputEncoding>
+<Url type="text/html" method="GET" template="https://www.example.org/">
+  <Param name="search" value="{searchTerms}"/>
+</Url>
+<SearchForm>https://www.example.org/</SearchForm>
+</SearchPlugin>
diff --git a/docshell/test/unit/data/enginePost.xml b/docshell/test/unit/data/enginePost.xml
new file mode 100644
index 000000000000..14775b6f0a0c
--- /dev/null
+++ b/docshell/test/unit/data/enginePost.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<SearchPlugin xmlns="http://www.mozilla.org/2006/browser/search/">
+<ShortName>test_urifixup_search_engine_post</ShortName>
+<Description>test_urifixup_search_engine_post</Description>
+<InputEncoding>UTF-8</InputEncoding>
+<Url type="text/html" method="POST" template="https://www.example.org/">
+  <Param name="q" value="{searchTerms}"/>
+</Url>
+<SearchForm>https://www.example.org/</SearchForm>
+</SearchPlugin>
diff --git a/docshell/test/unit/data/enginePrivate.xml b/docshell/test/unit/data/enginePrivate.xml
new file mode 100644
index 000000000000..7d87de98fa52
--- /dev/null
+++ b/docshell/test/unit/data/enginePrivate.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<SearchPlugin xmlns="http://www.mozilla.org/2006/browser/search/">
+<ShortName>test_urifixup_search_engine_private</ShortName>
+<Description>test_urifixup_search_engine_private</Description>
+<InputEncoding>UTF-8</InputEncoding>
+<Url type="text/html" method="GET" template="https://www.example.org/">
+  <Param name="private" value="{searchTerms}"/>
+</Url>
+<SearchForm>https://www.example.org/</SearchForm>
+</SearchPlugin>
diff --git a/docshell/test/unit/head_docshell.js b/docshell/test/unit/head_docshell.js
index c4603ae63e5a..6702a50b003c 100644
--- a/docshell/test/unit/head_docshell.js
+++ b/docshell/test/unit/head_docshell.js
@@ -13,6 +13,7 @@ ChromeUtils.defineESModuleGetters(this, {
 
 XPCOMUtils.defineLazyModuleGetters(this, {
   AddonTestUtils: "resource://testing-common/AddonTestUtils.jsm",
+  HttpServer: "resource://testing-common/httpd.js",
   NetUtil: "resource://gre/modules/NetUtil.jsm",
   TestUtils: "resource://testing-common/TestUtils.jsm",
 });
@@ -47,7 +48,6 @@ const SEARCH_CONFIG = [
 async function setupSearchService() {
   SearchTestUtils.init(this);
 
-  Services.prefs.setBoolPref("browser.search.modernConfig", true);
   AddonTestUtils.init(this);
   AddonTestUtils.overrideCertDB();
   AddonTestUtils.createAppInfo(
@@ -62,24 +62,39 @@ async function setupSearchService() {
   await Services.search.init();
 }
 
+/**
+ * After useHttpServer() is called, this string contains the URL of the "data"
+ * directory, including the final slash.
+ */
+var gDataUrl;
+
+/**
+ * Initializes the HTTP server and ensures that it is terminated when tests end.
+ *
+ * @param {string} dir
+ *   The test sub-directory to use for the engines.
+ * @returns {HttpServer}
+ *   The HttpServer object in case further customization is needed.
+ */
+function useHttpServer(dir = "data") {
+  let httpServer = new HttpServer();
+  httpServer.start(-1);
+  httpServer.registerDirectory("/", do_get_cwd());
+  gDataUrl = `http://localhost:${httpServer.identity.primaryPort}/${dir}/`;
+  registerCleanupFunction(async function cleanup_httpServer() {
+    await new Promise(resolve => {
+      httpServer.stop(resolve);
+    });
+  });
+  return httpServer;
+}
+
 async function addTestEngines() {
+  useHttpServer();
   // This is a hack, ideally we should be setting up a configuration with
   // built-in engines, but the `chrome_settings_overrides` section that
   // WebExtensions need is only defined for browser/
-  await Services.search.addPolicyEngine({
-    description: "urifixup search engine",
-    name: kSearchEngineID,
-    search_url: kSearchEngineURL,
-  });
-  await Services.search.addPolicyEngine({
-    description: "urifixup private search engine",
-    name: kPrivateSearchEngineID,
-    search_url: kPrivateSearchEngineURL,
-  });
-  await Services.search.addPolicyEngine({
-    description: "urifixup POST search engine",
-    name: kPostSearchEngineID,
-    search_url: kPostSearchEngineURL,
-    search_url_post_params: kPostSearchEngineData,
-  });
+  await SearchTestUtils.promiseNewSearchEngine(`${gDataUrl}/engine.xml`);
+  await SearchTestUtils.promiseNewSearchEngine(`${gDataUrl}/enginePrivate.xml`);
+  await SearchTestUtils.promiseNewSearchEngine(`${gDataUrl}/enginePost.xml`);
 }
diff --git a/docshell/test/unit/xpcshell.ini b/docshell/test/unit/xpcshell.ini
index 3a9b51f986e2..8bd7ef4f289e 100644
--- a/docshell/test/unit/xpcshell.ini
+++ b/docshell/test/unit/xpcshell.ini
@@ -1,5 +1,9 @@
 [DEFAULT]
 head = head_docshell.js
+support-files =
+  data/engine.xml
+  data/enginePost.xml
+  data/enginePrivate.xml
 
 [test_allowJavascript.js]
 skip-if = os == 'android'
diff --git a/toolkit/components/search/tests/xpcshell/head_search.js b/toolkit/components/search/tests/xpcshell/head_search.js
index 2c379e838412..9deb48c85106 100644
--- a/toolkit/components/search/tests/xpcshell/head_search.js
+++ b/toolkit/components/search/tests/xpcshell/head_search.js
@@ -45,8 +45,6 @@ var XULRuntime = Cc["@mozilla.org/xre/runtime;1"].getService(Ci.nsIXULRuntime);
 Services.prefs.setBoolPref("browser.search.log", true);
 Services.prefs.setBoolPref("browser.region.log", true);
 
-Services.prefs.setBoolPref("browser.search.modernConfig", true);
-
 AddonTestUtils.init(this, false);
 AddonTestUtils.createAppInfo(
   "xpcshell@tests.mozilla.org",