Starting with Firefox 45, the
performance.now method in workers will return a
DOMHighResTimeStamp from the worker’s creation time instead of the navigation start time inherited from the parent window or worker, as per the latest High Resolution Time spec. The new
Performance.translateTime method can be used to get a timestamp based on the main window context as before.
Update: This change has been reverted during the Firefox 45 Beta cycle because the spec seems unstable yet.