If you want to download a video from youtube you don’t have to use any third party software or browser plugin. Just go to www.youtube.com and search for a video of your choice. Once a video is playing, go to the address bar (just after http://www.) type ss (double s) without deleting anything and press enter key on your keyboard.
You will be redirected to savefrom.net, you will see the video you were playing and and on the right hand side under download links there is a list of links with their file type and quality for example (360p, 720p, 1080p). Click on any of the link to directly download the video.
Good luck using youtube.