hello, users of different downloading addons have reported similar issues with youtube recently - it's likely that youtube has implemented changes on their server-side that causes those extensions to break. please visit the website of the addon to inquire if there's already an updated version available or otherwise directly contact the developer of the addon and report the issue.

YouTube (Google) seems to have made changes to their servers once again and that prevent download extensions from working properly.

A lot of such extensions have been updated to fix this issue, so make sure to check if there is an update.

• Video DownloadHelper: https://addons.mozilla.org/firefox/addon/video-downloadhelper/

Otherwise you will have to wait until the extension developer has released an update to fix this or check for a possible beta version.