That's true, but at least Firefox born as FOSS so there is a community that potentially see the code growing by the time so while they may have lost something as individuals as a community there is a certain knowledge of the code. Webkit born open (KHtml, Kde's Konqueror html engine) but subsequent evolution happen inside few big companies and sources are release "en mass" so there is essentially "no community" that having at least seen the code grow a commit at a time so to have a sort of "big picture" knowledge...