Was failing on Fennec because private browsing doesn't exist there -- now more gracefully handles missing components.

services/sync/modules/service.js

@@ -650,7 +650,8 @@ WeaveSvc.prototype = {
       reason = kSyncNotLoggedIn;
     else if (Svc.IO.offline)
       reason = kSyncNetworkOffline;
-    else if (Svc.Private.privateBrowsingEnabled)
+    else if (Svc.Private && Svc.Private.privateBrowsingEnabled)
+      // Svc.Private doesn't exist on Fennec -- don't assume it's there.
       reason = kSyncInPrivateBrowsing;
     else if (Svc.Prefs.get("schedule", 0) != 1)
       reason = kSyncNotScheduled;

services/sync/modules/util.js

@@ -146,8 +146,15 @@ let Utils = {
   lazySvc: function Weave_lazySvc(dest, prop, cid, iface) {
     let getter = function() {
       delete dest[prop];
-      dest[prop] = Cc[cid].getService(iface);
-      return dest[prop];
+      if (!Cc[cid]) {
+        let log = Log4Moz.repository.getLogger("Service.Util");
+        log.warn("Component " + cid + " requested, but doesn't exist on "
+                 + "this platform.");
+	return null;
+      } else{
+        dest[prop] = Cc[cid].getService(iface);
+        return dest[prop];
+      }
     };
     dest.__defineGetter__(prop, getter);
   },