When you are watching a video on one these pages, Video DownloadHelper icon next to the address bar is activated, indicating the possibility of downloading videos from that page. Simply click on the video you want downloaded to start the process.
The list of sites supported by DownloadHelper Video is quite long. You can see if you click on the icon in the toolbar when it is idle.
In addition, Video DownloadHelper will allow you to download videos in AVI format or extract audio and save it to your MP3 file through FFmpeg.