How To Download YouTube Video's In Ubuntu Without Addons, apps, terminal etc

If you use Firefox or Chrome and want to "download" a YouTube video in Ubuntu you don't need to bother installing extensions, addons or 3rd party applications.

1. Go to the YouTube page of the video you want to download in Firefox
2. Let it load completely (be sure to select HQ/HD if you want best quality)
3. Do NOT close the browser OR the page
4. Open Nautilus and navigate to /tmp
5. Drag the file named "Flash" (followed by letters and numbers) to your desktop and rename it.
6. That's it!

If you use Google Chrome you will find the .flv's inside your ~./cache/google-chrome/Media Cache folder. 

You can play .flv's in VLC or you can now convert the file to a different format using your preferred method. Handbrake is pretty good!

Thanks to FRUMMAGE @ Ubuntuforums