[ Thanks to kiterunner for this link.
]
“How to download YouTube videos in Ubuntu is probably
among the first things a newbie Ubuntu user will ask for. There are
simply a number of application to do the same, but there is an even
better way. Downloading YouTube videos is Ubuntu is not the right
keyword, you can actually copy-paste the YouTube video. As simple
as that. Lets see how.“While watching YouTube videos in Ubuntu(in any browser), let
the video to buffer/load completely. Afterwards in your file
browser(nautilus), browse to the /tmp directory, like shown in the
screenshot below.”