diff --git a/testing/marionette/harness/marionette_harness/tests/unit/test_proxy.py b/testing/marionette/harness/marionette_harness/tests/unit/test_proxy.py
--- a/testing/marionette/harness/marionette_harness/tests/unit/test_proxy.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_proxy.py
@@ -61,16 +61,29 @@ class TestProxyCapabilities(MarionetteTe
             "socksProxy": proxy_hostname,
             "socksVersion": 4,
         }}

         self.marionette.start_session(capabilities)
         self.assertEqual(self.marionette.session_capabilities["proxy"],
                          capabilities["proxy"])

+    def test_proxy_type_manual_with_authentication(self):
+        proxy_hostname = "marionette.test"
+        username = "username"
+        password = "password"
+        capabilities = {"proxy": {
+            "proxyType": "manual",
+            "ftpProxy": "{}:{}@{}:21".format(username, password, proxy_hostname)
+        }}
+
+        self.marionette.start_session(capabilities)
+        self.assertEqual(self.marionette.session_capabilities["proxy"],
+                         capabilities["proxy"])
+
     def test_proxy_type_manual_socks_requires_version(self):
         proxy_port = 4444
         proxy_hostname = "marionette.test"
         proxy_host = "{}:{}".format(proxy_hostname, proxy_port)
         capabilities = {"proxy": {
             "proxyType": "manual",
             "socksProxy": proxy_host,
         }}
diff --git a/testing/marionette/session.js b/testing/marionette/session.js
--- a/testing/marionette/session.js
+++ b/testing/marionette/session.js
@@ -225,23 +225,29 @@ session.Proxy = class {
       if (!Number.isInteger(port)) {
         if (scheme === "socks") {
           port = null;
         } else {
           port = Services.io.getProtocolHandler(scheme).defaultPort;
         }
       }

-      if (url.username != "" ||
-          url.password != "" ||
-          url.pathname != "/" ||
+      if (url.username !== "") {
+        let login = Cc["@mozilla.org/login-manager/loginInfo;1"]
+                    .createInstance(Ci.nsILoginInfo);
+        login.init(`${scheme}://${url.hostname}`, null, url.hostname, url.username, url.password, "", "");
+        // dump(`Adding login for ${scheme}://${url.hostname}`);
+        Services.logins.addLogin(login);
+      }
+
+      if (url.pathname != "/" ||
           url.search != "" ||
           url.hash != "") {
         throw new InvalidArgumentError(
-            `${host} was not of the form host[:port]`);
+            `${host} was not of the form [username:password@]host[:port]`);
       }

       return [url.hostname, port];
     }

     let p = new session.Proxy();
     if (typeof json == "undefined" || json === null) {
       return p;
@@ -293,35 +299,41 @@ session.Proxy = class {
     return p;
   }

   /**
    * @return {Object.<string, (number|string)>}
    *     JSON serialisation of proxy object.
    */
   toJSON() {
-    function toHost(hostname, port) {
+    function toHost(scheme, hostname, port) {
       if (!hostname) {
         return null;
       }

+      let logins = Services.logins.findLogins({}, `${scheme}://${hostname}`, "", "", {});
+      if (logins.length) {
+        let login = logins[0];
+        hostname = `${login.username}:${login.password}@${hostname}`;
+      }
+
       if (port != null) {
         return `${hostname}:${port}`;
       }

       return hostname;
     }

     return marshal({
       proxyType: this.proxyType,
-      ftpProxy: toHost(this.ftpProxy, this.ftpProxyPort),
-      httpProxy: toHost(this.httpProxy, this.httpProxyPort),
+      ftpProxy: toHost("ftp", this.ftpProxy, this.ftpProxyPort),
+      httpProxy: toHost("http", this.httpProxy, this.httpProxyPort),
       noProxy: this.noProxy,
-      sslProxy: toHost(this.sslProxy, this.sslProxyPort),
-      socksProxy: toHost(this.socksProxy, this.socksProxyPort),
+      sslProxy: toHost("https", this.sslProxy, this.sslProxyPort),
+      socksProxy: toHost("socks", this.socksProxy, this.socksProxyPort),
       socksVersion: this.socksVersion,
       proxyAutoconfigUrl: this.proxyAutoconfigUrl,
     });
   }

   toString() { return "[object session.Proxy]"; }
 };