Firefox 17 is out of beta, but the biggest story for us Mac users is what it doesn't do, namely support OS X 10.5 Leopard. That's right, Mozilla's browser has moved on, so if you're still running 10.5, this latest version is not for you. But it does add a new "Social API," which Facebook has already used to bring an in-browser Messenger interface, and there are a number of new performance improvements and an "updated Awesome Bar experience" to check out as well.
Odds are that the 10.5 cutoff won't catch too many people any more, considering we've had three big OS updates since then (heck, Safari 6 doesn't run past 10.7.5). But if you are one of those with a Mac that still needs 10.5, you can of course keep running Firefox 16 for as long as you want. You just won't be able to update it any more going forward.