Firefox's Video Downloadhelper add-on:

Ever wondered how to download a video from Youtube?  

You're in luck.  The Firefox browser has an add-on called Video Downloadhelper available here that makes downloading videos a snap.  Once you've downloaded the add-on (no restart required), go to Youtube and find the video you need.  Video Download helper will automatically detect all available versions of the video.  To see this, click the colored ball animation in the upper right-hand corner of your browser window to see the list of videos.
 

Video Downloadhelper add-on shows all the Video versions to download

Typically you would choose the .mp4 version that is a higher resolution, e.g. Movie.mp4 at 1280x720p.  After clicking the Plus symbol next to the version you need, choose Download and save the video.  

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