If you click on the button, which is a downward pointing arrow next to the word "Download", you will see a selection list appear allowing you to select from the following options:
Next to each selection is the size of the file that will be produced if you select that option. In the example above, since a 720p MP4 file will be of a higher quality than a 360p one, it will occupy 27.8 MB of disk space whereas the 360p one will only consume 8.7 MB of space. Flash Video (FLV) is a container file format which is viewable on most operating systems via the Adobe Flash Player and web browser plugin or one of several third-party programs.
The extension, which is based on a Greasemonkey script, doesn't include ads and only links to YouTube's download URLs. The extension works in the regular YouTube interface or the HTML5 experiment. You don't need to restart Firefox after installing the add-on. The GitHub homepage for the extension is at https://github.com/gantt/downloadyoutube. The extension was originally created by Arpit Kumar.
After downloading a video, you can play it using a free application such as VLC, which is available for Microsoft Windows, Mac OS X, and Linux systems and which can play both MP4 and FLV videos. On a Microsoft Windows system, you can use the built-in Windows Media Player application to play MP4 files. You can also use iTunes or QuickTime Player from Apple, which are available for Microsoft Windows and Mac OS X sysems, to play MP4 videos.
Created: Saturday December 12, 2015