Mozilla Research is awesome. They punch way way way above their weight.
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.
You can't compare Google Research against Mozilla Research. Google is really moving computer science forward, just look at their papers and real implementations (disclaimer: I am not a Google fan).
If you compare the Google Chrome source code vs Firefox you will quickly understand how miles ahead Google is.
My opinion is that what Mozilla produces in the domain of programming languages is superior to what Google produces in the same domain. I make no claims comparing the two outside this domain.
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.
Google has been pretty influential in PL, especially in the PL/systems domain (MapReduce); just because they aren't designing so many mainstream languages doesn't mean people like Jeff Dean aren't putting their PL skillz to work. In fact, many of the heavy hitters at Google have PL backgrounds and do a lot of PL work, they are just pragmatic and realize that not all PL work involves overtly new languages.
And then there is what Holzle, Bak, Bracha, etc...are doing with Dart; and of course Pike with Go.
Chrome was started off of WebKit, which also has legacy from many years earlier. Both WebKit and Gecko have code that dates to before the year 2000 in fact.
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.)
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.