Greasemonkey is a Mozilla Firefox add-on that allows you, through code modifications, to change how certain websites work. What it does is allow the user to improve his browsing experience by adding functions or correcting errors.
Video downloads from YouTube built into your browser
It comes along with the peculiarity that we can add scripts to it, as is the case of All In One Video Script, which we can use to download videos from different sites such as YouTube or MetaCafe.
However, this plug-in has a drawback: the downloads can only be carried out in FLV format. For such purpose, just choose the Save as option and choose all files when you have to select the type.
You can only download videos in FLV format.
It's an option if you want to integrate the download of videos into your browsing experience using Firefox (forget about it if you use Chrome or Opera), but the truth is that there are better alternatives like aTube Catcher. Although at least you don't have to install anything with this script.