Firefox users can now upgrade to version 18 by downloading it via Mozilla's FTP server. A changelog isn't available yet as the official launch of Firefox 18 will follow later this week, but this beta release changelog should serve as a good guideline for what to expect in the latest Firefox edition:
NEW: Support for Retina Display on OS X 10.7 and up.
NEW: Preliminary support for WebRTC.
CHANGED: Experience better image quality with our new HTML scaling algorithm.
CHANGED: Performance improvements around tab switching.
DEVELOPER: CSS3 Flexbox implemented.
DEVELOPER: Support for new DOM property window.devicePixelRatio.
DEVELOPER: Support for @supports added.
DEVELOPER: Improvement in startup time through smart handling of signed extension certificates.
HTML5: Support for W3C touch events impemented, taking the place of MozTouch events.
FIXED: Disable insecure content loading on HTTPS pages (62178).
FIXED: Improved responsiveness for users on proxies (769764).