Why is he leaving? That's covered in the OP: he wants to devote his full-time attention to working on rr, a reversible debugger for native code that imposes impressively low overhead, and possibly spin it into a startup.
But what do I know, I'm just another ten-year Mozillian myself. And if or when I leave, it'll also surely be to get filthy rich at a startup, right?
It'd also be foolish to assume that most of those reasons are any of our business.
Robert is an amazing guy, and we're all lucky to have benefited from his work over the years. Now, he's set off on a new adventure, for reasons of his own... And has hinted that he'll continue sharing some of the fruits of his labor.
That's more than enough ;)