I assume you are talking about the [https://addons.mozilla.org/firefox/addon/video-downloadhelper/ Video DownloadHelper] add-on? If you are, it's usually best to reach out directly to the add-on developers for help. Only a few add-ons are actually developed and maintained by Mozilla. The others are owned by independent third-party developers. The developers offer [http://www.downloadhelper.net/support.php this website] for support with their add-on. Hope this helps.