Mozilla ok;s Extensions submitted by the Developers to be used with Firefox. They ok them only and give the Developer a web page to distribute their Extension from. This in no means means that Firefox Support does the Developers Support when their page https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/ has a Support link and a contact Link. Also do not know why it is not downloading. Firefox is working. Great. Try ant https://addons.mozilla.org/en-US/firefox/addon/video-downloader-player/ this is not a addon but may be useful https://fileforum.betanews.com/detail/VideoCacheView/1480357340/1 Please let us know if this solved your issue or if need further assistance.