--- a/toolkit/components/extensions/test/xpcshell/head.js
+++ b/toolkit/components/extensions/test/xpcshell/head.js
@@ -15,16 +15,20 @@ XPCOMUtils.defineLazyModuleGetters(this,
   ExtensionParent: "resource://gre/modules/ExtensionParent.jsm",
   ExtensionTestUtils: "resource://testing-common/ExtensionXPCShellUtils.jsm",
   FileUtils: "resource://gre/modules/FileUtils.jsm",
   HttpServer: "resource://testing-common/httpd.js",
   NetUtil: "resource://gre/modules/NetUtil.jsm",
   Schemas: "resource://gre/modules/Schemas.jsm",
 });
 
+XPCOMUtils.defineLazyServiceGetter(this, "proxyService",
+                                   "@mozilla.org/network/protocol-proxy-service;1",
+                                   "nsIProtocolProxyService");
+
 // These values may be changed in later head files and tested in check_remote
 // below.
 Services.prefs.setBoolPref("extensions.webextensions.remote", false);
 const testEnv = {
   expectRemote: false,
 };
 
 add_task(function check_remote() {
@@ -34,26 +38,56 @@ add_task(function check_remote() {
 
 ExtensionTestUtils.init(this);
 
 /**
  * Creates a new HttpServer for testing, and begins listening on the
  * specified port. Automatically shuts down the server when the test
  * unit ends.
  *
- * @param {integer} [port]
+ * @param {object} [options = {}]
+ * @param {integer} [options.port = -1]
  *        The port to listen on. If omitted, listen on a random
  *        port. The latter is the preferred behavior.
+ * @param {Set?} [options.hosts = null]
+ *        A set of hosts to accept connections to. Support for this is
+ *        implemented using a proxy filter.
  *
  * @returns {HttpServer}
  */
-function createHttpServer(port = -1) {
+function createHttpServer({port = -1, hosts} = {}) {
   let server = new HttpServer();
   server.start(port);
 
+  if (hosts) {
+    const serverHost = "localhost";
+    const serverPort = server.identity.primaryPort;
+
+    for (let host of hosts) {
+      server.identity.add("http", host, 80);
+    }
+
+    const proxyFilter = {
+      proxyInfo: proxyService.newProxyInfo("http", serverHost, serverPort, 0, 4096, null),
+
+      applyFilter(service, channel, defaultProxyInfo, callback) {
+        if (hosts.has(channel.URI.host)) {
+          callback.onProxyFilterResult(this.proxyInfo);
+        } else {
+          callback.onProxyFilterResult(defaultProxyInfo);
+        }
+      },
+    };
+
+    proxyService.registerChannelFilter(proxyFilter, 0);
+    registerCleanupFunction(() => {
+      proxyService.unregisterChannelFilter(proxyFilter);
+    });
+  }
+
   registerCleanupFunction(() => {
     return new Promise(resolve => {
       server.stop(resolve);
     });
   });
 
   return server;
 }

--- a/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_filterResponseData.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_webRequest_filterResponseData.js
@@ -1,44 +1,17 @@
 "use strict";
 
-XPCOMUtils.defineLazyServiceGetter(this, "proxyService",
-                                   "@mozilla.org/network/protocol-proxy-service;1",
-                                   "nsIProtocolProxyService");
-
-const server = createHttpServer();
-const gHost = "localhost";
-const gPort = server.identity.primaryPort;
-
 const HOSTS = new Set([
   "example.com",
   "example.org",
   "example.net",
 ]);
 
-for (let host of HOSTS) {
-  server.identity.add("http", host, 80);
-}
-
-const proxyFilter = {
-  proxyInfo: proxyService.newProxyInfo("http", gHost, gPort, 0, 4096, null),
-
-  applyFilter(service, channel, defaultProxyInfo, callback) {
-    if (HOSTS.has(channel.URI.host)) {
-      callback.onProxyFilterResult(this.proxyInfo);
-    } else {
-      callback.onProxyFilterResult(defaultProxyInfo);
-    }
-  },
-};
-
-proxyService.registerChannelFilter(proxyFilter, 0);
-registerCleanupFunction(() => {
-  proxyService.unregisterChannelFilter(proxyFilter);
-});
+const server = createHttpServer({hosts: HOSTS});
 
 server.registerPathHandler("/redirect", (request, response) => {
   let params = new URLSearchParams(request.queryString);
   response.setStatusLine(request.httpVersion, 302, "Moved Temporarily");
   response.setHeader("Location", params.get("redirect_uri"));
   response.setHeader("Access-Control-Allow-Origin", "*");
 });