Hello,
In Firefox there is a plugin https://addons.mozilla.org/en-US/firefox/addon/3006 called downloadhelper http://www.downloadhelper.net/
Just install this firefox plugin and ensure that you have the ffmpeg package installed through your distro's package manager. When you play the video in youtube you will an animated tool icon next to the address bar. It will allow you directly download and convert the video to your customised format, even mp3 for sound only. In the Tools -> DownloadHelper menu there is a 'preferences' section. Thats where you first set all your custom preferences and once you are set, simply click on the drop down list of the animated tool icon and select a download video format, it will open another submenu that has options like 'download', 'download and convert' etc. Thats it. It is a one click download and convert to your preferred folder.