TUAW posted a story that Firefox 17 has dropped OS X 10.5 support

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.

  Firefox 17 drops OS X 10.5 Leopard support, adds integrated Facebook Messenger