It is not yet ready. Expected for Fx 28 (but would not sign it with my blood). There is a UX Nightly with Australis (the name of the redesign) out there.
Really looking forward to it. Maybe it makes me sound superficial but that might be what finally makes me switch back from Chrome - the compactness of the Chrome UI is great. Firefox on OSX is really in need of a boost.
I don't know how well it works on a OS X, but I have been using the firefox-ux nightly build on Windows for awhile now. You can find them at [0]. It's been as stable for me as the other nightly builds tend to be, which is very stable, but do be aware that going this route can lead to running into bugs. I understand why these things take a long time to shake out, but it is massively disappointing to hear that the new UI won't be released until Firefox 28 or later!
