I've been using the latest Firefox Beta pretty much exclusively now for about a week. It's been surprisingly stable and a bit faster as well.
But it also has a surprising new feature: tear-off tabs. This is something that Safari's had over Firefox for some time, and it's something I've actually longed for. It's often very convenient to be able to tear off a tab into its own window.
And better still is the ability to tear off a tab and put it into the tab bar of another window. Finally! This is awesome!
Another minor, but still cool, new feature of Firefox 3.1 is that middle-clicking a tab in a window with only a single tab open will actually close the tab and, thus, the window. Previously, middle-click did nothing in such a circumstance.