It's been public knowledge for a while now that Mozilla intends to future versions of Firefox to have a uniform look across all form factors.
But now Jared Wein, a Software Engineer at Mozilla, has written a blog post with an image of the new Australis design (or the new curvy tab shape, anyway). And if that's not enough, he's also created a test build (Windows-only) so that you can view it for yourself.
Wein says the aim of Australis is to provide "more customizability in a cleaner and fresher user interface", and it's certainly looking good to us.
However, if the grabs aren't enough then you can always download a copy to try it further. We're linking to the ZIP file version here, you can just unzip and run it without overwriting your current Firefox installation (but this is a test build, so we wouldn't use it for anything other than a quick look at the revamped tabs).