Tabs haven’t changed a whole lot between Firefox 2 and Firefox 3 except for the addition of new smooth scrolling animation. When you scrolled your tab bar in Firefox 2, the tabs moved back and forth a full tab at a time. This made scrolling a bit choppy and disjointed. With smooth tab scrolling, it’s much easier to understand how the movement is happening, and where tabs are moving to and from. This is most clearly demonstrated with a demo movie, so I created a quick one which you can see here: Firefox 3: Smooth tab scrolling (.swf).
