Firefox Australis was supposed to be done by now, as the plan was to have the new revamped interface ready for Firefox 24. That obviously hasn't happened; the Australis UI isn't even enabled by default in the current Nightly Firefox 27 builds.
So it's no surprise that Mozilla finally admitted that the UI revamp was not ready. The latest official goal is Firefox 28, which is going stable on March 4 next year.
Australis has been pushed back so many times now that it's hard to be confident that Firefox 28 will be the one that makes the big switch.
But, if that happens, we should be seeing the new UI become the default in the Nightly and Aurora channels this year.
The Mozilla user experience team has been putting out official Australis builds for several weeks now. The new UI is usable, but there are still plenty of performance and stability issues.
These are getting ironed out and shouldn't take that much longer, especially since the bulk of the work has been done.
Still, if things go according to plan (they haven't so far), Firefox 28 with Australis enabled will be hitting the Nightly channel on November 1. The UI may be turned on by default at any time during the Nightly cycle, which lasts six weeks.
Mozilla won't switch to the new UI if there is a performance regression. Right now, there is a visible difference between the current UI and Australis, for the worse.
That said, Firefox purists are already complaining about the Australis look, saying that it resembles Google Chrome too much. Purists have a tendency to complain about a lot of things and it may very well be that once the new look goes live, only a few people will be upset. But that's still something Mozilla has to be ready for.