The browser now displays razor-sharp text and imagery on new-gen Macs
The official changelog (part of which you can see displayed in the screenshot above) clarifies that the Retina goodness only applies to OS X 10.7 or higher Mac OS installations.
Also worth noting for Mac users, the latest Stable version of Firefox (17.0) no longer supports OS X 10.5 Leopard. For those running newer versions of the Mac OS, Firefox 17 and newer (beta / alpha) versions can be grabbed via the link below.
Download Mozilla Firefox for Mac OS X (Free)