This is a bit on the “old news” side of things, but have you heard of SeaMonkey? SeaMonkey is FireFox’s smaller, uglier but faster brother.
SeaMonkey is a web browser from the Mozilla Foundation that’s based on Gecko – the same “layout engine” as FireFox. That means it renders HTML to your screen the exact same way as FireFox does.
While FireFox uses the fancy, but “expensive”, XUL for its user interface, SeaMonkey uses the native UI elements of whatever OS it runs on. SeaMonkey has less features and looks uglier but runs faster that FireFox and consumes considerably less system resources. SeaMonkey now has versions for Windows, Linux and OS X.
SeaMonkey is no FireFox. I wouldn’t use it as my default browser here at work on my powerful desktop station. But on occasions where you have limited CPU or memory resources, it’s very useful. Its quicker response times are highly appreciated on slower machines.
Followup to original post: Thanks to all commenters. I stand corrected: SeaMonkey also uses XUL. Its faster-than-FireFox performance can be attributed to having less features written in leaner code.