why do you want to download it and it’s already on your machine ?! 😀
Just go to YouTube or Facebook and watch the video for the first time (make sure it’s all streamed)
DO NOT refresh or close the browser (I tested it with Firefox and Chromium)
then navigate on you Ubuntu machine to the path “/tmp/”, and find the video there named “Flashxxxx” Ta Daaaa
if you can’t find it check this post
copy it and rename it to whatever you want and append the extension “.flv”
then you can watch it, or even put it in a folder you wont open again 😀
the same thing tested with Facebook videos.