YouTube Download How-To
Unfortunately the flash player plugin for firefox doesn't support JACK, so if you want to playback a YouTube video through jack you will have to download it.
The testing branch of 64 Studio now sports the youtube-dl package, which you can install with synaptic or from the terminal with apt-get.
To download a video simply copy and past its URL on the command line:
free@64studio:~ $ youtube-dl "http://www.youtube.com/watch?v=AB8VA1lAvPY"
Just don't forget the to place the URL between doublequotes.