After using it for a day, one more thing drove me batty about Firefox 2. With more than a few tabs open, they started scrolling all over the place, making it extremely difficult to jump back and forth between them.
Here's how to turn the tab scrolling feature off:
* Go to about:config
* Filter for the word tab
* set browser:tabs:tabMinWidth to 0
Maybe I'm stuck in my old ways, but I use tabbed browsing a LOT and so far, I don't count these two changes as improvements. I do love the dropdown list of tab names on the right though!