The evolution of the web platform. In the past fifteen months, Firefox has added at least the following major features:
* Web fonts
* Vast improvements in SVG support
* Out of process plugins, to stop Flash crashing your browser all the time (which required a major internal rearchitecture)
* Profile sync
* Large chunks of HTML5 and CSS3
* Rendering system changes to support mobile and GPU-accelerated graphics
You can't do this with small patches on top of a stable base.