As I tweeted recently, Firefox 4 is to the 3.x line what Windows 7 is to Windows XP. It really feels like a worthy successor in so many ways. Tabs on top is a great enhancement, and I especially like the tabs-in-the-title-bar approach. I'm really able to maximize my screen real estate with these options. Surprisingly, I don't miss the status bar or menu bar as much as I thought I would (and yes, I know the menu bar is still present; I've simply chosen to turn it off). The orange "Firefox button" is a little strange, and takes some getting used to, but I can live with it.
The biggest improvement in my eyes is the ability to pin certain sites as "app tabs." Currently, I have GMail and Twitter pinned open. I am the world's worst at closing Firefox down completely at various points during the day. I don't know why I do this, but knowing I have some app-tabs open will hopefully help me break this terrible habit. One other great improvement worth mentioning is start-up time, which is notably faster. They've really caught up to (though not surpassed) Chrome in this regard, which has always been lightning fast to boot up. Hopefully this trend will continue.