Downloading video works very sporadically with Firefox 56. Downloading mp3 audio is fine. Developer made a solid effort to help, but the add-on doesn't work with FF56 for video in most circumstances. More recent versions of Firefox appear to have no problems. Updated: works perfectly on FF88 on another laptop. This app is updated regularly, so I donated to upgrade to the unrestricted version. A must for grabbing YouTube vids. Now 5 stars and highly recommended.