Most of the time I have two instances of my browser open: one for Youtube and one for all the other stuff.
As for tabs: I like branching, which means I open the home page of a website and if there are links I want to open, it happens in a new tab. That makes navigating pretty fast. If I'm done with, for example writing a post I close the tab and am allready on the next tab which holds the forum with the thread I posted in before. Also most of the time my "root tabs" contain some search results from google.