--- a/dom/base/nsPerformance.cpp
+++ b/dom/base/nsPerformance.cpp
@@ -496,17 +496,21 @@ nsPerformance::Navigation()
     mNavigation = new nsPerformanceNavigation(this);
   }
   return mNavigation;
 }
 
 DOMHighResTimeStamp
 nsPerformance::Now() const
 {
-  return GetDOMTiming()->TimeStampToDOMHighRes(TimeStamp::Now());
+  double nowTimeMs = GetDOMTiming()->TimeStampToDOMHighRes(TimeStamp::Now());
+  // Round down to the nearest 0.005ms (5us), because if the timer is too
+  // accurate people can do nasty timing attacks with it.
+  const double maxResolutionMs = 0.005;
+  return floor(nowTimeMs / maxResolutionMs) * maxResolutionMs;
 }
 
 JSObject*
 nsPerformance::WrapObject(JSContext *cx, JS::Handle<JSObject*> aGivenProto)
 {
   return PerformanceBinding::Wrap(cx, this, aGivenProto);
 }