Firefox 15 supports SPDY networking protocol v3, WebGL enhancements including compressed textures, native support for the Opus audio codec. Apart from those mentioned, there's nothing really qualifies "awesomeness" in this release.
Firefox 15 is available through self-update (if the user has not touched the default setting) and available from Firefox downloading page. Firefox 15 still supports Windows XP SP2 (released in August 2004) on Windows platform, on the Mac end, it's a bit unfair that Firefox only supports Intel-based Mac with OS X Leopard (10.5) (released in October 2007) or later on Mac platform.
UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:15.0) Gecko/20100101 Firefox/15.0 AlexaToolbar/alxf-2.15