Because Chrome and Firefox’s plug-in architecture is effectively neutered in the interest of security (mostly because Windows security is poor), I have not been able to find a good youtube download plugin. The plugins that exist under the new interface require a third party website be involved and those seem limited and unreliable.
So because some things that make it to youtube don’t stay there long, owing to Google’s censorship policy, I find the need to have a reliable downloader. I have installed clibgrab on Mint, Julinux, Ubuntu, and Zorin. It is under Applications->Internet->Clipgrab.