Edit: Recent link: youvlc.xpi
This has probably been done already. But with the neat ability to play youtube videos right from the URL, I thought it would be nice to have Firefox open youtube URLs directly in VLC. I played around with extensions a little, no idea if this is any good.
The extension adds a button to start VLC for the current video being watched, and a contextmenu item to open Youtube URLs in VLC (or download them).