Mozilla is working on a lot of things at the same time, some vaguely related to the browser that it's best known for. It's working on standardizing web APIs, on a new browser for the iPad, on better ways to learn programming and so on.
On the Firefox front, updates have been constant but the steady stream of small new features makes it seem like there's not much going on. But there is one major change on the way, a brand new UI.
Granted, it's just a reimagining of the current one, but it's completely redesigned. It's called Australis and it's coming, though it's hard to pinpoint when.
Until it's ready though, we get a small teaser if you will, a peek at how the rounded tabs will look like, at least on Windows. You can see the evolution from the initial Australis design mockups.