Well, sure the kernel does a better job of it. The kernel not only does a better job of it than Firefox, it does a better job of it than any other project in the history of mankind.
Why? Well, re-read the first half of the sentence you quoted... the kernel has, overall, extraordinarily clean code as its starting point. Plus, the kernel has fewer users, more developers, less coupling (most kernel code is standalone device drivers!), and vastly simpler exposed interfaces.
None of which is to say that Firefox is anywhere close to perfect, and it may well yet be overtaken by Webkit etc. But many of the people slagging on Firefox seem to be patting themselves on the back over how incredibly clever they are to have misdiagnosed Firefox's failure to live up to their misguided expectations.