Further performance improvements touch on tab switching and startup time (through smart handling of signed extension certificates).
Preliminary support for WebRTC has been added, and thanks to a new HTML scaling algorithm, users should experience better image quality. Mac users, however, also benefit from HiDPI support, which makes Firefox look clearer on machines with Retina Display.
As far as HTML5 is concerned, the fresh revision comes with W3C touch events, which replace MozTouch events.
Starting Firefox 18 Animated Personas will no longer work. However, there is an easy fix for this by installing Personas Shuffler add-on. If more than one persona is installed, the add-on will activate a different one each time the web browser starts.
Firefox 18 was on a 7-week release cycle because of the holidays, but the next releases will be available at the regular 6-week interval.