Firefox 13 is available through self-unstoppable-update and available from Firefox downloading page. If you are in limited and expensive internet connection (e.g. hotel internet), this self-update might cost you a bit. Firefox 13 still supports Windows XP SP2 (released in August 2004) on Windows platform, and only supports Intel-based Mac with OS X Leopard (10.5) (released in October 2007) or later on Mac platform.
If you notice anything significant in Firefox 13, please do share with us.