Bug 962709 - Fix about:home's sync entry point to work with about FxAccounts r=markh

browser/modules/AboutHome.jsm

@@ -15,6 +15,8 @@ Components.utils.import("resource://gre/modules/Services.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "PrivateBrowsingUtils",
   "resource://gre/modules/PrivateBrowsingUtils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "fxAccounts",
+  "resource://gre/modules/FxAccounts.jsm");
 
 // Url to fetch snippets, in the urlFormatter service format.
 const SNIPPETS_URL_PREF = "browser.aboutHomeSnippets.updateUrl";
@@ -146,7 +148,21 @@ let AboutHome = {
         break;
 
       case "AboutHome:Sync":
-        window.openPreferences("paneSync");
+        let weave = Cc["@mozilla.org/weave/service;1"]
+                      .getService(Ci.nsISupports)
+                      .wrappedJSObject;
+
+        if (weave.fxAccountsEnabled) {
+          fxAccounts.getSignedInUser().then(userData => {
+            if (userData) {
+              window.openPreferences("paneSync");
+            } else {
+              window.loadURI("about:accounts");
+            }
+          });
+        } else {
+          window.openPreferences("paneSync");
+        }
         break;
 
       case "AboutHome:Settings":