Google is chock full of programming language people, but Mozilla's work on Rust and Javascript (ES6, asm.js, sweet.js) is producing far better languages than Google has done with Go and Dart in my humble opinion.
If you compare the Google Chrome source code vs Firefox you will quickly understand how miles ahead Google is.
Google has far more resources than Mozilla, and produces loads of great stuff in many domains.
I use both browsers daily (Aurora specifically, for Firefox) and have no preference between them. I have no opinion on their source code.
And then there is what Holzle, Bak, Bracha, etc...are doing with Dart; and of course Pike with Go.
This is pretty much expected for a huge multimillion line C++ codebases, like all web browsers currently are. All have crufty parts (not sure why GP thinks one browser has nicer code overall? That's not my opinion based on the code I've read.)
WebKit forked KHTML in 2005.
KHTML actually started in late '98 if memory serves, when we rebooted Mozilla around Gecko.
/be