How can i download any video on the spot like with teh realplayer extension?

realplayer has an extension with which you can download any video you want, for example when you're watching a youtube video a tab appears on top of the video that says download this video...but now for some reason on firefox it doest work anymore...is there another kind of extension i can use or program to download videos from ANY website right on the spot like the realplayer extension?