[summerlight]

You severely underestimate the engineering cost of modern web browser. Assuming a sufficient value-addition fork, a team of 20 cannot even catch up the Chromium upstream. Good luck coming up with a new engine compatible with Chrome; MS tried it and finally gave up.

[ItsBob]

Not sure where the Chromium part comes in but I'm talking about Firefox here.

Anyway, all the heavy-lifting is done: The JS engine, the CSS engine and so on.