--- a/netwerk/protocol/http/UserAgentOverrides.jsm
+++ b/netwerk/protocol/http/UserAgentOverrides.jsm
@@ -9,21 +9,25 @@ this.EXPORTED_SYMBOLS = [ "UserAgentOver
 const Ci = Components.interfaces;
 const Cc = Components.classes;
 
 Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
 Components.utils.import("resource://gre/modules/Services.jsm");
 Components.utils.import("resource://gre/modules/UserAgentUpdates.jsm");
 
 const PREF_OVERRIDES_ENABLED = "general.useragent.site_specific_overrides";
-const DEFAULT_UA = Cc["@mozilla.org/network/protocol;1?name=http"]
-                     .getService(Ci.nsIHttpProtocolHandler)
-                     .userAgent;
 const MAX_OVERRIDE_FOR_HOST_CACHE_SIZE = 250;
 
+// lazy load nsHttpHandler to improve startup performance.
+XPCOMUtils.defineLazyGetter(this, "DEFAULT_UA", function() {
+  return Cc["@mozilla.org/network/protocol;1?name=http"]
+           .getService(Ci.nsIHttpProtocolHandler)
+           .userAgent;
+});
+
 var gPrefBranch;
 var gOverrides = new Map;
 var gUpdatedOverrides;
 var gOverrideForHostCache = new Map;
 var gInitialized = false;
 var gOverrideFunctions = [
   function (aHttpChannel) { return UserAgentOverrides.getOverrideForURI(aHttpChannel.URI); }
 ];