--- a/testing/marionette/server.js
+++ b/testing/marionette/server.js
@@ -7,17 +7,16 @@
 const CC = Components.Constructor;
 
 const ServerSocket = CC(
     "@mozilla.org/network/server-socket;1",
     "nsIServerSocket",
     "initSpecialConnection");
 
 ChromeUtils.import("resource://gre/modules/Log.jsm");
-ChromeUtils.import("resource://gre/modules/Preferences.jsm");
 ChromeUtils.import("resource://gre/modules/Services.jsm");
 
 ChromeUtils.import("chrome://marionette/content/assert.js");
 const {GeckoDriver} = ChromeUtils.import("chrome://marionette/content/driver.js", {});
 const {WebElement} = ChromeUtils.import("chrome://marionette/content/element.js", {});
 const {
   error,
   UnknownCommandError,
@@ -36,18 +35,16 @@ const {KeepWhenOffline, LoopbackOnly} = 
 
 this.EXPORTED_SYMBOLS = ["server"];
 
 /** @namespace */
 this.server = {};
 
 const PROTOCOL_VERSION = 3;
 
-const PREF_PORT = "marionette.port";
-
 /**
  * Bootstraps Marionette and handles incoming client connections.
  *
  * Starting the Marionette server will open a TCP socket sporting the
  * debugger transport interface on the provided <var>port</var>.
  * For every new connection, a {@link server.TCPConnection} is created.
  */
 server.TCPListener = class {
@@ -109,17 +106,17 @@ server.TCPListener = class {
    */
   start() {
     if (this.alive) {
       return;
     }
 
     // Start socket server and listening for connection attempts
     this.acceptConnections = true;
-    Preferences.set(PREF_PORT, this.port);
+    MarionettePrefs.port = this.port;
     this.alive = true;
   }
 
   stop() {
     if (!this.alive) {
       return;
     }