How To Download YouTube Video's In Ubuntu Without Addons, apps, terminal etc
"Go to the YouTube page of the video you want to download in Firefox
Let it load completely (be sure to select HQ/HD if you want best quality)
Do NOT close the browser OR the page
Open Nautilus and navigate to /tmp
Drag the file named 'Flash' (followed by letters and numbers) to your desktop and rename it.
That's it!
If you use Google Chrome you will find the .flv's inside your ~./cache/google-chrome/Media Cache folder."