Phoenix/Firefox was Mozilla's own lightweight alternative to SeaMonkey. Then Chrome was the lightweight alternative to a memory-hogging Firefox. Now Chrome is seen as sluggish and a memory hog. Where do we go next?
I suppose, as far as the web platform is concerned, we aren't really going to replace HTML/CSS/JS so increasing the speed and memory management of javascript rendering is the clear way forward, e.g. with Servo https://www.mozilla.org/en-US/research/projects/