Changelogs: http://www.mozilla.org/en-US/firefox/18.0beta/releasenotes/
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).