The update enables MacBook Pro with Retina display owners to use an alternative to Safari or Google Chrome, and enjoy the benefits of retina display rendering.
Mac OS X converts program menus and windows into Retina Display, but the program itself has to be re-written to support Retina graphics. In the case of Firefox the new beta enables it to render text and graphics from websites at Retina resolutions, for sharper text and a more visually pleasant viewing experience. To take full advantage of the Retina display, websites still have to render graphics at the Retina level.
The Firefox 18 Beta also has preliminary support for WebRTC, CSS3 Flexbox, @supports, W3C touch events, and a number of fixes and updated algorithms for performance improvements.