There are many ways to download videos from YouTube. But there is no direct link at YouTube for downloading video.You required some other third party tool to download.
Here is a simple and very practical method which adds to YouTube what is it lacking - links to download the video in different formats offered, and the ability to copy the direction of the download link video.You just required a simple script called YouTube DL Tool installed on yor Firefox. To do this firstly install Greasemonkey extension from Mozilla add-on site, then install YouTube DL in to Geasemonkey.
After the installation, you can see the direct link to download the video from YouTube, as seen in the image, in different format offered.
Install Geasemonkey from here, then Install YouTube DL Tool from here.