Msujaws has posted a build of Mozilla Firefox featuring the new Australis tabs. This feature is not currently available on the regular Firefox channels.
Mozilla is expected to roll out this new updated design of tabs in their regular builds in the coming months.
Msujaws spoke about the new tabs:
Wanna play with a sneak peek of the new Australis tabs for Firefox? I’ve put together a build of it that you can download and run with. This isn’t checked in yet, but it should be coming along soon. I only implemented it for Windows, so my apologies go out to those on OS X or Linux.