--- a/services/sync/modules/resource.js
+++ b/services/sync/modules/resource.js
@@ -69,30 +69,16 @@ AsyncResource.prototype = {
   /**
    * Callback to be invoked at request time to add authentication details.
    *
    * By default, a global authenticator is provided. If this is set, it will
    * be used instead of the global one.
    */
   authenticator: null,
 
-  // The string to use as the base User-Agent in Sync requests.
-  // These strings will look something like
-  //
-  //   Firefox/4.0 FxSync/1.8.0.20100101.mobile
-  //
-  // or
-  //
-  //   Firefox Aurora/5.0a1 FxSync/1.9.0.20110409.desktop
-  //
-  _userAgent:
-    Services.appinfo.name + "/" + Services.appinfo.version +  // Product.
-    " FxSync/" + WEAVE_VERSION + "." +                        // Sync.
-    Services.appinfo.appBuildID + ".",                        // Build.
-
   // Wait 5 minutes before killing a request.
   ABORT_TIMEOUT: 300000,
 
   // ** {{{ AsyncResource.headers }}} **
   //
   // Headers to be included when making a request for the resource.
   // Note: Header names should be all lower case, there's no explicit
   // check for duplicates due to case!
@@ -156,18 +142,17 @@ AsyncResource.prototype = {
     channel.loadFlags |= DEFAULT_LOAD_FLAGS;
 
     // Setup a callback to handle channel notifications.
     let listener = new ChannelNotificationListener(this.headerNames);
     channel.notificationCallbacks = listener;
 
     // Compose a UA string fragment from the various available identifiers.
     if (Svc.Prefs.get("sendVersionInfo", true)) {
-      let ua = this._userAgent + Svc.Prefs.get("client.type", "desktop");
-      channel.setRequestHeader("user-agent", ua, false);
+      channel.setRequestHeader("user-agent", Utils.userAgent, false);
     }
 
     let headers = this.headers;
 
     if (this.authenticator) {
       let result = this.authenticator(this, method);
       if (result && result.headers) {
         for (let [k, v] in Iterator(result.headers)) {

--- a/services/sync/modules/rest.js
+++ b/services/sync/modules/rest.js
@@ -23,40 +23,24 @@ this.SyncStorageRequest = function SyncS
 }
 SyncStorageRequest.prototype = {
 
   __proto__: RESTRequest.prototype,
 
   _logName: "Sync.StorageRequest",
 
   /**
-   * The string to use as the base User-Agent in Sync requests.
-   * These strings will look something like
-   * 
-   *   Firefox/4.0 FxSync/1.8.0.20100101.mobile
-   * 
-   * or
-   * 
-   *   Firefox Aurora/5.0a1 FxSync/1.9.0.20110409.desktop
-   */
-  userAgent:
-    Services.appinfo.name + "/" + Services.appinfo.version +  // Product.
-    " FxSync/" + WEAVE_VERSION + "." +                        // Sync.
-    Services.appinfo.appBuildID + ".",                        // Build.
-
-  /**
    * Wait 5 minutes before killing a request.
    */
   timeout: STORAGE_REQUEST_TIMEOUT,
 
   dispatch: function dispatch(method, data, onComplete, onProgress) {
     // Compose a UA string fragment from the various available identifiers.
     if (Svc.Prefs.get("sendVersionInfo", true)) {
-      let ua = this.userAgent + Svc.Prefs.get("client.type", "desktop");
-      this.setHeader("user-agent", ua);
+      this.setHeader("user-agent", Utils.userAgent);
     }
 
     if (this.authenticator) {
       this.authenticator(this);
     } else {
       this._log.debug("No authenticator found.");
     }
 

--- a/services/sync/modules/service.js
+++ b/services/sync/modules/service.js
@@ -1238,17 +1238,17 @@ Sync11Service.prototype = {
     if (ignore && ignore.indexOf(reason) != -1)
       return "";
 
     return reason;
   },
 
   sync: function sync(engineNamesToSync) {
     let dateStr = new Date().toLocaleFormat(LOG_DATE_FORMAT);
-    this._log.debug("User-Agent: " + SyncStorageRequest.prototype.userAgent);
+    this._log.debug("User-Agent: " + Utils.userAgent);
     this._log.info("Starting sync at " + dateStr);
     this._catch(function () {
       // Make sure we're logged in.
       if (this._shouldLogin()) {
         this._log.debug("In sync: should login.");
         if (!this.login()) {
           this._log.debug("Not syncing: login returned false.");
           return;

--- a/services/sync/modules/util.js
+++ b/services/sync/modules/util.js
@@ -55,16 +55,35 @@ this.Utils = {
   makeHMACKey: CryptoUtils.makeHMACKey,
   makeHMACHasher: CryptoUtils.makeHMACHasher,
   hkdfExpand: CryptoUtils.hkdfExpand,
   pbkdf2Generate: CryptoUtils.pbkdf2Generate,
   deriveKeyFromPassphrase: CryptoUtils.deriveKeyFromPassphrase,
   getHTTPMACSHA1Header: CryptoUtils.getHTTPMACSHA1Header,
 
   /**
+   * The string to use as the base User-Agent in Sync requests.
+   * This string will look something like
+   *
+   *   Firefox/49.0a1 (Windows NT 6.1; WOW64; rv:46.0) FxSync/1.51.0.20160516142357.desktop
+   */
+  _userAgent: null,
+  get userAgent() {
+    if (!this._userAgent) {
+      let hph = Cc["@mozilla.org/network/protocol;1?name=http"].getService(Ci.nsIHttpProtocolHandler);
+      this._userAgent =
+        Services.appinfo.name + "/" + Services.appinfo.version +  // Product.
+        " (" + hph.oscpu + ")" +                                  // (oscpu)
+        " FxSync/" + WEAVE_VERSION + "." +                        // Sync.
+        Services.appinfo.appBuildID + ".";                        // Build.
+    }
+    return this._userAgent + Svc.Prefs.get("client.type", "desktop");
+  },
+
+  /**
    * Wrap a function to catch all exceptions and log them
    *
    * @usage MyObj._catch = Utils.catch;
    *        MyObj.foo = function() { this._catch(func)(); }
    *
    * Optionally pass a function which will be called if an
    * exception occurs.
    */

--- a/services/sync/tests/unit/test_resource_ua.js
+++ b/services/sync/tests/unit/test_resource_ua.js
@@ -2,16 +2,19 @@
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
 Cu.import("resource://services-sync/constants.js");
 Cu.import("resource://services-sync/resource.js");
 Cu.import("resource://services-sync/service.js");
 Cu.import("resource://services-sync/util.js");
 Cu.import("resource://testing-common/services/sync/utils.js");
 
+var httpProtocolHandler = Cc["@mozilla.org/network/protocol;1?name=http"]
+                          .getService(Ci.nsIHttpProtocolHandler);
+
 // Tracking info/collections.
 var collectionsHelper = track_collections_helper();
 var collections = collectionsHelper.collections;
 
 var meta_global;
 var server;
 
 var expectedUA;
@@ -32,17 +35,20 @@ function run_test() {
   });
 
   ensureLegacyIdentityManager();
   setBasicCredentials("johndoe", "ilovejane");
   Service.serverURL = server.baseURI + "/";
   Service.clusterURL = server.baseURI + "/";
   _("Server URL: " + server.baseURI);
 
+  // Note this string is missing the trailing ".destkop" as the test
+  // adjusts the "client.type" pref where that portion comes from.
   expectedUA = Services.appinfo.name + "/" + Services.appinfo.version +
+               " (" + httpProtocolHandler.oscpu + ")" +
                " FxSync/" + WEAVE_VERSION + "." +
                Services.appinfo.appBuildID;
 
   run_next_test();
 }
 
 add_test(function test_fetchInfo() {
   _("Testing _fetchInfo.");

--- a/services/sync/tests/unit/test_service_getStorageInfo.js
+++ b/services/sync/tests/unit/test_service_getStorageInfo.js
@@ -2,16 +2,19 @@
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
 Cu.import("resource://services-common/rest.js");
 Cu.import("resource://services-sync/constants.js");
 Cu.import("resource://services-sync/service.js");
 Cu.import("resource://services-sync/util.js");
 Cu.import("resource://testing-common/services/sync/utils.js");
 
+var httpProtocolHandler = Cc["@mozilla.org/network/protocol;1?name=http"]
+                          .getService(Ci.nsIHttpProtocolHandler);
+
 var collections = {steam:  65.11328,
                    petrol: 82.488281,
                    diesel: 2.25488281};
 
 function run_test() {
   Log.repository.getLogger("Sync.Service").level = Log.Level.Trace;
   Log.repository.getLogger("Sync.StorageRequest").level = Log.Level.Trace;
   initTestLogging();
@@ -32,16 +35,17 @@ add_test(function test_success() {
     do_check_eq(error, null);
     do_check_true(Utils.deepEquals(info, collections));
 
     // Ensure that the request is sent off with the right bits.
     do_check_true(basic_auth_matches(handler.request,
                                      Service.identity.username,
                                      Service.identity.basicPassword));
     let expectedUA = Services.appinfo.name + "/" + Services.appinfo.version +
+                     " (" + httpProtocolHandler.oscpu + ")" +
                      " FxSync/" + WEAVE_VERSION + "." +
                      Services.appinfo.appBuildID + ".desktop";
     do_check_eq(handler.request.getHeader("User-Agent"), expectedUA);
 
     server.stop(run_next_test);
   });
   do_check_true(request instanceof RESTRequest);
 });

--- a/services/sync/tests/unit/test_syncstoragerequest.js
+++ b/services/sync/tests/unit/test_syncstoragerequest.js
@@ -3,30 +3,34 @@
 
 Cu.import("resource://gre/modules/Log.jsm");
 Cu.import("resource://services-sync/constants.js");
 Cu.import("resource://services-sync/rest.js");
 Cu.import("resource://services-sync/service.js");
 Cu.import("resource://services-sync/util.js");
 Cu.import("resource://testing-common/services/sync/utils.js");
 
+var httpProtocolHandler = Cc["@mozilla.org/network/protocol;1?name=http"]
+                          .getService(Ci.nsIHttpProtocolHandler);
+
 function run_test() {
   Log.repository.getLogger("Sync.RESTRequest").level = Log.Level.Trace;
   initTestLogging();
 
   ensureLegacyIdentityManager();
 
   run_next_test();
 }
 
 add_test(function test_user_agent_desktop() {
   let handler = httpd_handler(200, "OK");
   let server = httpd_setup({"/resource": handler});
 
   let expectedUA = Services.appinfo.name + "/" + Services.appinfo.version +
+                   " (" + httpProtocolHandler.oscpu + ")" +
                    " FxSync/" + WEAVE_VERSION + "." +
                    Services.appinfo.appBuildID + ".desktop";
 
   let request = new SyncStorageRequest(server.baseURI + "/resource");
   request.onComplete = function onComplete(error) {
     do_check_eq(error, null);
     do_check_eq(this.response.status, 200);
     do_check_eq(handler.request.getHeader("User-Agent"), expectedUA);
@@ -36,16 +40,17 @@ add_test(function test_user_agent_deskto
 });
 
 add_test(function test_user_agent_mobile() {
   let handler = httpd_handler(200, "OK");
   let server = httpd_setup({"/resource": handler});
 
   Svc.Prefs.set("client.type", "mobile");
   let expectedUA = Services.appinfo.name + "/" + Services.appinfo.version +
+                   " (" + httpProtocolHandler.oscpu + ")" +
                    " FxSync/" + WEAVE_VERSION + "." +
                    Services.appinfo.appBuildID + ".mobile";
 
   let request = new SyncStorageRequest(server.baseURI + "/resource");
   request.get(function (error) {
     do_check_eq(error, null);
     do_check_eq(this.response.status, 200);
     do_check_eq(handler.request.getHeader("User-Agent"), expectedUA);