I have found Video DownloadHelper indispensable over the years, especially when the browser won't play a video with sound for some reason. It is very useful for cases where you can't get a URL that youtube-dl can use. In some cases it can display a URL that you can download directly with wget or curl, as an alternative to downloading using the plugin. It's a great workaround when you want a video and the site doesn't provide a way for people to download (i.e., streaming only).